Who is to say that it isn't working as intended in building a higher wall around their ad garden? There should be a way for them to accept mail from self hosted users, or even provide some sort of testing tools (perhaps validated by credit card some type of bar to increase accountability? At least an RBL or something) for those who do, but nope.

I am beginning to think that the simplest explanation is the most likely one. Google is just bad at stuff...

If this was all part of some clever conspiracy then it would be, well, more clever...

Or more simple still, it's a hard problem to solve and when there's billions of emails flowing through the pipeline every hour small mistakes can look big to individual users.

Same as with gaming search, it’s possible that spammers are catching up with the Google smarts, and from this point it’s unclear how to proceed.

"Never attribute to malice that which is adequately explained by stupidity", except for the "promotions tab", that was clear malice to increase their ads revenue.

I don't think they're bad, it feels more like they hit the 80/20 point and move on to something else. In the case of spam detection, whatever worked ten years ago set and forget without anyone quality assuring it as things evolve.

Also, they want more people to use Gmail or GSuite. If that's the way to reach an Inbox on GMail, people simply choose the path of least resistance.