It's interesting to follow opinions on this matter as they seem to change over the years like fashion. The 1950s "Golden Age" of Sci-Fi took it as writ that we were destined to master Faster Than Light (FTL) travel and march into the stars, with the vision peaking in Star Trek: The Next Generation. But after then the views have gotten a lot more pessimistic. Kim Stanley Robinson chronicled the nitty-gritty challenges we'd face in Terraforming Mars, but in his more recent Aurora has argued that colonizing outside our own solar system is downright impossible. And then there's Cixin Liu with his Three Body Problem series, showing that as soon as we poke our heads over the surface of the galactic waters, some other species is bound to swoop down and gobble us up.
I'm not really sure what I believe, but with matters this big, I think the opinions say more about human psychology than they do about alien life.
Yeah, different people have different views and that is totally understandable. I just don't think we should cast aside something as "impossible" though.
People who say things are impossible are my favourite kind because I like to see the look on their faces when the same things are made possible right in front of their eyes!!