The trouble is knowing that it isn't likely doesn't stop it from also being true. I'm also of the opinion that just because full communism isn't likely doesn't mean we shouldn't advocate for it, because any move toward liberty, freedom, equality, and the general principles of anarchism and socialism are good things. You don't come to the table with your compromise, you come to the table with what you know you can't get, and negotiate to something possible.
Do I believe communism is possible within my lifetime? No.
Do I believe it possible at all? Absolutely, not only in the sense that if we did it it would work, but that we can and likely will do it, eventually, if we survive long enough.
Do I believe it's worth fighting for, even if I'll never see it? Yes. Because the work itself is enough to improve lives, and the more people who throw their lot in with the far left the more likely we are to see real, substantive change for the better, even if it is incremental.