I mostly read non-fiction and won't try speaking to the novel market, but in other pop culture media, it seems what happens is more the long tail of the reasonably good, still has artistic value middle that hollows out. There's always going to be a high end, if for no other reason than sufficiently rich people are often willing to lose money patronizing great art for its own sake, but the vast bulk of new content becomes mass-produced generic trash with easily replaceable creatives who are given no real creative control. I'd say it happened to mass-market film somewhere between 2005 and 2010. It's probably happening to serial filmed content right now, thanks to the zero-interest streaming wars being over and peak TV coming to an end. It arguably happened to pop music as early as the 1980s, but music is comparatively cheap to produce and starving artists will always self-finance, just discovering their work is difficult when it's made outside of a recording industry marketing machine.

Novels seem like they should have the same dynamic. It's effectively free to write novels. You only pay with time and stress and don't need money. But without a publisher to market you, the hope that anyone will ever read what you write gets dimmer and dimmer.

In the same vein, I feel like serial cartoons are holding out much better right now than filmed television. They're still in a golden era, probably because they're so much cheaper to produce that they can survive not being popular without getting canceled.