Pretty sure there have been a few spam bot/latent bot purges over the last month on multiple instances. I remember people discussing them a little while back. And it was a ton of bots.
So part of what you’re seeing is normalization, as the user numbers were always artificially inflated with bots.
Honestly, Google did this to themselves with not properly vetting the advertisers that they sell space to, and with oversaturation of ads.
If they’d have stopped granting ad space to scammers and malware spreaders, and if they’d have stopped adding advertisements at the line most people find tolerable (which seems to be a single ad between videos… not multiple at a time, and certainly no mid-rolls), they wouldn’t have triggered quite the level of ad blocking that they did.
I see this “problem” that they have as being entirely of their own making.