Given that there are only so many people editing Wikipedia and only so many hours in the day, it seems trivially obvious to me that "better" necessarily implies "fewer".

Umm, no. That only follows if the editors are assumed to be spending significant amounts of their time editing the pages that would be deleted.

Getting rid of trivial articles (which are usually also of lousy standard) raises the mean quality of articles on Wiki; it does nothing to raise the quality of any individual article that remains. Yes, deleting the trivial articles would theoretically narrow the number of articles that the editors need to fix, but I think the argument of the inclusionists is that the editors should be concentrating on the important articles anyway. Leave the trivia to quietly rot on its own; put in a Wiki rule that a page that isn't visited for x time gets deleted or somesuch.

The "average" quality of Wiki articles is irrelevant; what's important is the quality of the articles that people (other than the creators) actually read. The quality of those articles is affected very little by the quantity of trivial vanity pages.

If you hit the "random" button, you should expect trivia; that's the way the world is.