Well, while the present mayor's trying, Boston City Hall hasn't been given the chance yet to "meet its fate"--not because it isn't dysfunctional, but that it'd involve a lot of thinking and effort (and municipal resources) to blow it up and start over. And heck, it's still less than 40 years since it opened--so it's jumping the gun to assume that "overriding merit" is directly behind its continued existence (even if I'm prone to defending it). It's still there because they're hogtied.
Outside of archi-geeks, it's *very* hard to tell about Brutalism--likewise, going the other direction, with the retro-schlock condo/McMansion aesthetic; it may be more "appealing" to the masses, but is it the sort of appeal that'll draw preservation-minded sentimentality down the line? Or will it be more ashes-to-ashes? After all, even now, an Ron Thom or Arthur Erickson home of the 1970s is more likely to inspire rallies than, say, a Napier Simpson home of that same era (and that was *tasteful* retro). Likewise, I assume in the future, Shim-Sutcliffe vs Gordon Ridgeley.
Perhaps, ultimately, the whole "brutalism will have its day" or "you never know; today's McMansions might become tomorrow's landmarks" arguments might simply be consumed by the increasing segmentation of taste and eclipse of consensus. Once upon a time, it was accepted that the chronological march of judgment of heritage-worthiness was inexorable; today, with something like
Neutra's Cyclorama at Gettysburg, the debate takes on more of a Red/Blue America never-the-twain-shall-meet tinge...