"So this is a huge win for us, the developers."
There's a contradiction here, and I'm not being cute. Your argument is not the only argument to be made, and personally, as a developer, I preferred the old way.
Yep, there are negative externalities, and that always has to be weighed, but I think it was a reasonable approach to allow developers to weigh the use of cutting edge features against their own communities, needs, and goals. That calculation would change depending on how many browsers were offering a proposed recommendation and the browser proportions of your viewers. Lots of interesting stuff was posted around the web, including here, taking advantage of these things. On the other hand, there's virtually no community for whom it would be reasonable to ask to make config changes to view a site, so you you simply can't use cutting edge features. Experimental site designs will be harder to show off and we'll have less public consideration of the implications of new recommendations since there's much less joy in putting together projects that few people will see.
Showing off demos leveraging experimental browser features that don't happen to be CSS features has often been done with requests to make config changes, because the "put it behind a config flag until it is ready" is pretty standard for everything other than CSS (and, actually, browser vendors have also done it for plenty of experimental implementations of CSS, whether or not the features themselves are standard or vendor-prefixed experimental extensions.)
Actually, HN is exactly the sort of place where people might be willing to do so. Things are posted here which require specific browsers or about:flags changes in Chrome, what would change?