Quite right on the first 3! I use to have his 3 axioms as my tag line. He was way beyond his time in most respects (but so are mot SciFi Authors, especially Isaac Asimov, my favorite.
But on 4, I think some probably are sentient. It is just that they are probably not much more advanced than we are - so they cannot get here, or we are just not interesting enough yet.
Either we aren't interesting enough, or they can't get here, or they are freaking scared. I'm serious with the 3rd one. It's not that they're scared of us, or what we might become.
It's that they're afraid of making too much noise, and getting noticed by something they can't compete with. In short, they're afraid of being smashed apart by something bigger and stronger and nastier. My personal opinion on sentient non-Earth-life is, that if it's out there, and it's as advanced or more advanced than we are, then they are not friendly.
They are mean, and they are going to get us before we get them. IF they fire first. Hence, the dilemma; do you fire, do away with a possible threat, but in so doing get noticed by a third party?
To use an example that is shamelessly ripped off of a favorite website of mine:
"....We ask that you try just one more thought experiment. Imagine yourself taking a stroll through Manhattan, somewhere north of 68th street, deep inside Central Park, late at night. It would be nice to meet someone friendly, but you know that the park is dangerous at night. That's when the monsters come out. There's always a strong undercurrent of drug dealings, muggings, and occasional homicides.
It is not easy to distinguish the good guys from the bad guys. They dress alike, and the weapons are concealed. The only difference is intent, and you can't read minds.
Stay in the dark long enough and you may hear an occasional distance shriek or blunder across a body.
How do you survive the night? The last thing you want to do is shout, "I'm here!" The next to last thing you want to do is reply to someone who shouts, "I'm a friend!"
What you would like to do is find a policeman, or get out of the park. But you don't want to make noise or move towards a light where you might be spotted, and it is difficult to find either a policeman or your way out without making yourself known. Your safest option is to hunker down and wait for daylight, then safely walk out.
There are, of course, a few obvious differences between Central Park and the universe.
There is no policeman.
There is no way out.
And the night never ends."- Atomic Rockets, Aliens Page, Fermi Paradox subsection, The Killing Star excerpt.
The thought experiment/argument given above is based on 3 (with a 4th amendment given by a third party) ideas (paraphrased from the link given above):
1. 'Their' survival is more important than 'Our' survival. In other words, if it comes down to us or them, they'll choose themselves.
2. Wimps' don't become Top Dogs. An intelligent, top-of-the-food-chain species (like humans, for example) isn't passive. They're ruthless and aggressive. That's how they made it to the top.
3. 'They' will assume the above two rules (or assumptions) apply to 'Us'.
4. Technology implies belligerence. In a nutshell, I've taken this to mean any species which has attained space travel, and particularly STL (or even FTL, if it's possible) interstellar travel, isn't going to be a nice 'guy'. Effectively, they'll be mean, and they'll be out to protect their own interests in any way possible.
The trade-off? They aren't going to actively attempt to attack. They'll disperse, and stay quiet. Dispersal means that they maximize species survival chances, and staying quiet means they don't attract attention. This is, incidentally, why we haven't found any signs of intelligent life (if it's out there): it's afraid, so it stays quiet.
Anyone who buys into a 'friendly' universe theory assumes that we humans have to conquer our self-destructive tendencies in order to not wipe ourselves out. Not a bad assumption in and of itself. The fallacy they fall into is that they assume that any species which attains space travel of any kind on a large scale will be like that. In other words, 'as we are down here, so they are above'.
The fallacy is that 'they' are not part of our species. Any alien species is 99% or more likely to be difficult, if not impossible, to understand. They will not be like us, at all.