The impossibility of interstellar travel is perhaps the only fact that I find so depressing that I simply refuse to believe it. The universe is just so much more interesting if FTL is possible. It helps that being wrong about this fact doesn't really impact one's life either way.
Okay, then I'll cheer you up just a bit: interstellar travel is quite possible. In fact it's easy... if you can cold hibernate.
It would be very easy for an AI that could simply turn itself off for tens of thousands of years. It might be a do-able for an alien biological entity with a different biology that finds it easier to survive cryogenic deep freeze, something like a big smart tardigrade.
Managing sleep and wake is pretty easy too. No internal timing mechanisms needed. Just let your proximity to a star do it for you. When you leave the vicinity of a star you have no energy and you go night-night. Your temperature will fall down to as low as a few kelvin. This basically stops time in terms of any chemical degradation, though the most sensitive bits will need to be wrapped in radiation shielding against cosmic rays to prevent cumulative damage over the aeons. When you approach another star the temperature starts to go up and solar power will power everything up.
Interstellar travel is only hard for us because we have short life spans and can't survive being frozen. Unless we can figure out how to successfully freeze a human down to liquid helium temperatures and then revive them it's unlikely that humans will ever leave the solar system.
The AIs we build might though. This would include "uploaded" minds if we ever figure out how to do that.
If anything ever does visit us my money would be on it being an AI.
If you find humans being stuck in the solar system depressing, consider that the solar system is absolutely gigantic and we could explore and settle it for aeons and aeons without getting close to exhausting that frontier.
Imagine a religious order like the space Mormons in the Expanse creating a self replicating evangelism bot. A few billion years later nothing in the galaxy with more brain power than a mouse can get through life without being asked if it wants to hear about another testament of Jesus Christ.
We do help seed millions of other advanced civilizations though since when they become smart enough they are able to learn a ton by reverse engineering some of the millions of dead probes littering their planet and every other celestial body. Their first messages into the cosmos are variations of “okay which one of you bastards did this?”
Nobody is ever able to figure out the meaning of the probe’s message and humanity is long extinct.
Not meaning to diss Mormons though. If anyone built a generational ship I can imagine them doing it.
You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space. ~ Douglas Adams
Space is really big. Really, really big.