I think the chance that we are the only sentient species in the entire universe is negligible. The only question is: Does it live long enough?
My rather pessimistic theory about why we didn't even found traces of an alien civilization is that most(maybe even all) sentient live will at some point die because of wars/asteroids/stupidity(which we can observe on our own planet)/… before it gets the opportunity to literally reach for the stars.

He haven't had much technological advance in space travel since the cold war. We didn't build the warp drive (yet) and we are not even able to accelerate faster than a few g if a human sits inside. So I think a bit alien help can't hurt. I don't think they would invade us. Our planet (as it will look after a destructive war )offers nothing that cannot be found anywhere else in the galaxy. So no species that is intelligent enough to discover space travel will waste their precious resources on us.

And even if they'd invade us for some other reason, I wouldn't see that as a problem. It doesn't matter if humanity(humans are nothing more than some stupid creatures that claim being intelligent because they are more intelligent than every species they know of) survives, it only matters that sentience, regardless of its form, preferably in form of the more intelligent species, survives.