When I say it's "we could start a colony at Alpha Centauri with current or conceivable technology", wiggin, what I mean it is not actually physically impossible for us to do it, not that I think such an undertaking is likely or even a good idea.

If aliens showed up tomorrow and issued an ultimatum: "This is getting ridiculous now; If don't at least have a colony on Alpha Centauri by 2200 we're going to exterminate you. You're making the whole galaxy look bad." and we just devoted like 25% of the entire planet's GDP to making it happen, it could be done. That's what I mean, not that I think a project like that would happen under any realistic economic and political constraints.

But the fact that it's a theoretical possibility even now has implications for assessing how difficult it it would be for a vastly more advanced society than our own.

The other caveat is that it all rather depends on the presence of an Earthlike planet within approximately 20 lightyears with a reasonably non-hostile biosphere. No point going to Alpha Centauri to colonize a hellhole when we have so many hellholes much closer to hand.

If propulsion gets dramatically better, it will at least be feasible, even if ruinously expensive, to set up a colony. But absent a lot of other fundamental shifts in technology, there's no way it's going to appear to be worth it.
I can't really envisage a future where we get vastly better propulsion but all other technologies stay exactly the same. Now *that* would be science fiction, which is a genre where, apparently, future societies have mastered the reality bending physics of FTL travel, and the engineering challenges involved in constructing miles long space warships but everyone's day to day lives are more or less identical to the present day, perhaps even absent day to day conveniences like smartphones or the internet. (Somewhat off-topic but it's a pet peev so fuck it)