I don't think anyone here is saying that you're wrong, only that you can't discard the idea based solely on risk.
Nor am I doing so. I'm not saying we shouldn't colonize space or mine asteroids; I'm just trying to explain how the incentives currently available don't motivate the kinds of efforts people think are cool space colonies. There is, I think, a marked tendency for people to just identify someone who could possibly do the grandiose science thing they think is cool and blame them for not doing it because they're greedy and stupid and evil, which I know feels good to say but isn't really helpful. The next step after "we should do X thing" needs to be to ask why we aren't, not to find the nearest entity notionally capable of doing it, presuppose why they aren't doing it, and call them nasty names.
The Columbus analogy is less than perfect for several reasons, but the basic motivation for the trip would have been good had his math been sound: fund expedition, possibly get direct sea route to the East along which caravels can immediately go to supply a demonstrable need for the quantities of spices they can carry. Everything was in place except that route, which is also the sort of scenario where science funding becomes available: the benefits are not much less concrete than the costs and there's a small number of unknown points of failure.
The Columbus equivalent to asteroid mining would have been asking Isabella to fund the development of carvel-hulled ships and shipyards capable of producing them so that ships capable of transatlantic crossings could be developed, a route found, and spices brought back as above, all without any concrete plan for what to do if any of those things do not work as expected. It's too many steps with too many unknown unknowns to predict, so of course nobody's throwing money at it as such. Even the much-heralded Planetary Resources isn't launching asteroid miners. It's launching for-hire space telescopes. DSI has been acquired by Bradford Space, but when confronted about the extreme cost of their proposals they quickly backpedaled to servicing communications satellites.
So no, I'm not saying we should discount the idea based on the risk. I'm saying that airy proclamations about how space is our future and protects us from extinction and it's the most important thing we could be doing and so forth don't actually fly, pun intended, because the risk is still incalculably high. Incrementally decreasing that risk requires intermediate applications to motivate the requisite technology development and establish a need for space infrastructure, but unfortunately those are complicated and boring and don't lend themselves as readily to complaining on the Internet, so everyone's going to keep being disappointed for a while.