Given that at least that much is crap, and a large chunk consists of stuff set up for no other purpose than to lure in ad revenue, shrinkage here may not be a bad thing.
At the very least it might get advertisers and those depending on them for revenue finally thinking about how to reach people. Traditionally it's by snatching your attention in the most annoying way possible because any exposure is good advertising, right?
And then you get autoplaying videos, or animated gifs, blinking tags, flashing flash, or whatever else they'll think up next. In short, "dancing rodents", in advertising flavour.
I block things when they annoy me and when I do I block everything on the page. That's using an ad-blocker, though without the prefab lists, I just grow my own. So if advertisers want their advertisements to stay visible, well, they better make sure the advertisements do not annoy me.
Annoying includes posing as real content only turning out to be vapid and snickering, having succeeded at wasting my time (adwords, say). Or as simple as burning too many cycles with js, ajax, whatever, especially when the real content could've been served up js-free. "Pingers" that track my eyeballing the site and phoning home every second get booted with prejudice.
Advertisers need to re-think, since "fighting" the audience for their attention has become a lot less useful because the audience can fight back, and rightfully so. For am I the product, or a "consumer" with no other rights than to "consume"? I don't think so.
The key to good business is to add value, and merely screaming loudest you're the best, really, is not adding value. Marketing needs to grow up.