I won't play semantics with you so I'll give you hard numbers. There are roughly 200 billion stars in our galaxy, and Keplar Space Mission data estimates roughly 40 billion Earth-sized planets in habitable zones orbiting sun-like or red-dwarf stars in the Milky Way Galaxy. So there are roughly 40 billion Earth-sized planets out of 100 billion. Stealing from a different source, in May of 2018, the chief curator of the Habitable ExoPlanets Catalog said that roughly 53 out of 3,730 known (discovered) exoplanets in that habitable zone are capable of supporting life. Key word there is capable of. So we've got a baseline of roughly 1.4% of planets in the habitable zone could potentially support life, or roughly 570 million planets in our galaxy are capable of supporting life. I won't get into the philosophical/religious arguments about us possibly being the only fucking planet in the entire universe capable of supporting life. For the purpose of my argument, we'll say roughly 0.001% of planets that could contain life have intelligent life on them, or something greater than bacteria anyways, not even talking about spacefaring civilization, just life capable of one day fostering civilization. The number could be much smaller, but this number works just fine for the point I'm trying to make here even though it's astronomically larger than it should be. This would mean roughly 5,700 planets in the Milky Way Galaxy might have multi-cellular organisms on them. Cool. Whatever. The Milky Way Galaxy is 1,000,000,000,000,000,000km user. That's 1 quintrillion km in diameter. Surface Area is A=π*(ø/ 2)^2 and that's assuming the milky way is flat (which it's not). I won't do that calculation because it's a fuck huge number. It's 1 quintrillion times itself times pi. You're looking for an earth-sized object that's roughly 13,000km across. Just working with 2D numbers you're at less than 0.0000000000013% of the surface of the galaxy ignoring all the planets and solar systems in the way, and if we add even 10km of space in the Z axis it becomes exponentially lower. Lasers would get blotted out by other planets, signals would get blotted out by other planets. The chances of encountering life are actually LOWER than the chances of two bullets being shot perfectly so that they ricochet off each other, but it's so insignificantly low, that the chances of a single individual winning the lottery 50 times is probably more likely (I won't do the actual math for that one either, fuck you).
Space and Extraterrestrial Warfare General
As for your second point, EM waves have exponential decay and can be confused by shit like stars that also let out "EM waves." Have you ever heard of a pulsar? It's quite annoying- there's roughly 200,000 of them in our galaxy and they're basically the Milky Way's big FUCK YOU when we're out looking for those mystical "EM waves" and "Lasers." Turns out physics is a dick and most things we'd consider signs of artificial structures/intelligent life are almost always caused by some cluster of gases naturally occurring or some star somewhere releasing some energy or radio signal because it bounced off of something. Probability is a bitch in space- in fact probability is the only reason stars (which are not nearly dense enough for nuclear fusion to take place if you use Newtonian Physics) don't instantly burn up when they react.
Trying to hit a planet in another solar system with a rocket would be like trying to hit a man on Earth with a red ryder BB gun fired from the Kuiper Belt
That still leaves a lot of debris in orbit after the war, and even more the more effective the higher the interception rate of the program. Unless you're planning to plant 'orbital mines' in the form of several tonnes of metal scrap directly above your opponents launch facilities it seems a little counterproductive.
So all it takes is a single god-tier marksman (from an entire species, mind you) or a supercomputer and several hours/days of calculation?
There's a difference between being dark and staying put, you can still leave your planet and colonize other places you just can't spam EM signals everywhere along the way. If not everyone agreed to not send signals then your civilization gets wiped out, so you don't see the civilizations that fail to do this.
Why do you think 0.001% of planets supporting life is low? How do you know that life is not statistically inevitable on a fertile planet? You can't know unless you have a sample size greater than 1. And again, we aren't looking for an earth-sized object, we're looking for suspicious EM waves which spread outwards at the speed of light.
It's true that we have trouble distinguishing artificial signals from noise, but why should you assume that our current ability to do this will never improve? Technology improves exponentially, and any discussion about alien life that doesn't account for this is meaningless.
This is just stupid. Any civilization that is in any way like ours would not "go dark" forever because of a theoretical danger. It just wouldn't. It'd be like expecting Spaniards not to go colonising America because "Dude, what if that continent over yonder is full of incredibly powerful demons that will come back to Spain to eat us all?" Even if some decided to go through with this (I mean why should all of these gorillion civilizations have one world government? There would be dissent if it ain't some hivemind), others would not, and not bothering with all that shit would offer them competitive advantage.
Mankind needs to be rebuilt at the molecular level before we can conquer the galaxy. With genetic engineering we can make ourselves stronger, faster, hardier, and smarter. That much is obvious. But we will need to do more than strengthen our muscles, arm ourselves with best guns and clad ourselves in the best armor. We mus defend ourselves from foreign germs that would kill us. The best solution would be a bacteriophage that is programmed to attack any bacteria (excluding helpful ones in your gut), as well as parasites.
Please try reading before replying.
Of course new civilizations will pop up that will likely not go dark, but they won't last very long so it would be unlikely that we would have seen their signals in the short amount of time that we've been looking (plus our instruments still suck).
I had a weird question about alien life. Shouldn't life-bearing planets be most likely to be found on the rim of a galaxy because of the large concentration of giant stars in more coreward regions putting out more starlight that will stop the development of life on coreward or mid-galactic planets? If this is the case, it rules out the overwhelming majority of possible planets as candidates for the development or settlement of Earth-like life by itself. The higher gravity of a location further into a galaxy also means more frequent occurrence of hazardous space objects like asteroids that will destroy civilizations before they can become advanced even if they somehow survive a high-starlight environment. Earth is on the rim of the Milky Way IIRC.
In terms of what I expect for the reasonably near future of space combat, it's probably going to be mostly battles of space stations versus other space stations because of orbital dynamics making certain points important enough to put a permanent space base at as shows with the Lagrange points. If you take a more long-term view of space habitats, space stations rely way too much on local planets and stars for supplies, are vulnerable to Kessler syndrome, and can't get out of the system when the star dies because they don't have enough supplies to make it to another system, and engineered systems have the risk of unstable orbits the more planets you have. Neither is suitable for a permanent habitat. The best way of creating a sustainable spacefaring civilization by my reckoning is equipping your star with a Shkadov thruster so you can pack up and move your entire solar system elsewhere to escape a cosmic threat and collect fuel for your star. Space conflicts in the very distant future would primarily be between planets in mobile solar systems using planetary-scale weapons on each other to decide who gets to refuel their star with otherwise unused celestial bodies.