That's not how this would work.
Based on previous history where people actually did call google's bluff to their regret, what happens is that google trusts all current certificates and just stops trusting new certs as they are issued.
Google has dragged PKI security into the 21st century kicking and screaming. Their reforms are the reason why PKI security is not a joke anymore. They are definitely not afraid to call CA companies bluff. They will win.
How is "client certificates forbidden" in any way an improvement?
Not forbidden, just not going to be a part of WebPKI.
It's one of those things that has just piggybacked on top of WebPKI and things just piggybacking is a bad idea. There have been multiple cases in the past where this has caused a lot of pain for making meaningful improvements (some of those have been mentioned elsewhere in this thread).
What exactly do you mean with "WebPKI"?
The PKI system was designed independently of the web and the web used to be one usecase of it. You're kind of turning that around here.
The current PKI system was designed by Netscape as part of SSL to enable secure connections to websites. It was never independent of the web. Of course PKIs and TLS have expanded beyond that.
"WebPKI" is the term used to refer to the PKI used by the web, with root stores managed by the browsers. Let's Encrypt is a WebPKI CA.
The idea of a PKI was of course designed independently, there are many very large PKIs beyond WebPKI. However the one used by browsers is what we call WebPKI and that has its own CAs and rules.
You're trying to make it sound like there has ever been some kind of an universal PKI that can be used for everything and without any issues.