To answer the question, my point is that there are countless examples of powerful, inventive, playful, varied works of modernist+contemporary architecture. I simply reject the premise. I brought up the Barbican because the core of his argument is "nobody likes these things because they're ugly," and I'm saying: I like them. They move me. Some of my most enduring early memories are of the Barcellona Pavillion and the Getty Center. And to read these architects, that was often the point. I guess I'm asking, why should I give a shit about this guy's taste? Don't yuck my yum.
He's just not putting forward a very strong case, nor invoking others who have studied these questions before him. I'm not claiming that there aren't any valid critiques, just that his isn't one of them. If he's only stating that middling traditionalist buildings look worse than middling modern ones: I buy that, and I think it's probably a question of economics, as he says. But to me, when recent architects set out to create the monolithic and meaningful, it's not clear they're less successful than their predecessors.
Regarding public tastes, a mea culpa: I had dismissed the survey too quickly, and on a deeper look it's reasonable (although it includes few instances of what I'd call great 20th+21st century examples). But, as the Civic Art Society says, it's hard to disentangle Americans' aesthetic preferences with their normative conception of what a federal building (or any important building) should look like. That will naturally tend toward traditional styles. I'm also not convinced that today's choice expression of populist preferences---the McMansion---is going to stand any test of time. Whereas I do think currently dispreferred styles will endure and come to signal beauty.
As for writing, I guess I found Le Corbusier's treatises convincing enough? But for defenses that post-date the modernist era, I'm not sure I have an answer for you. To me, the best defense is the work itself.