SAME. I was skimming headlines before bed and this woke me the fuck up before I read it properly.

It went from an in-industry only trade show where they could basically show off to other devs and bounce ideas off each other to nothing more than a dolled-up hype train.

What ruined it was over commercialization and making it a public event, turning it from an developer conference to a gigantic fucking ad.

Livestreaming becoming commonplace was a big part of it. Why would a brand go through the expense of setting up a presentation and booth in E3 when they can name the where, when and how, in a much more controlled fashion, for much cheaper, to reach more eyes directly.
The primary purpose of E3 for them was as a marketing space so when a more convenient form of doing it appeared, it was done.

I still remember E3 as "the weeks when all gaming Youtube channels went to shit" because they all went to E3 and instead of their usual high quality content they had all kind of crap content about games that weren't even out yet and wouldn't be for years in some cases.

I'd wager it's mostly big studios leaving E3 and holding their own dedicated presentations