Of course, nearly 100% of the times the behaviour people were complaining about will be part of the stable release. There is no magical moment where things just magically get sorted pre-release unless people voice feedback, especially if it's intended behaviour, as it's very much the case here. So call me jaded, when someone says "it's just the main branch, don't complain!" I just hear "I will do whatever I want and when the new release rolls in everyone will just have to deal with it." Which is totally fair if that's how you want to run your project, just don't pretend otherwise.
An extra one, which isn't the case here but makes me roll my eyes every times it happens is the outrage of "how dare you raise an issue that was caused by a new pre-release iOS/MacOS version and very much seems to be an intended change in the OS behaviour, we don't support that!!", only for it to turn into the usual scramble "oh no, our software is broken on the new iPhones, who could have forseen this!!?" hours after the release version starts hitting the masses.
I agree with @tarsius on this: just give the discussion (and patches) some more time, and it's likely to end just OK from past experience.
Ultimately, the consequences of a patch, especially one that changes UX, can only really be evaluated after the community starts using it. People using the master branch of Emacs are basically those who wish to work as the QA engineers of Emacs. End-users use release branches or even pre-packaged releases from their distro.
So, the normal process for a UX change is to merge it to master in order to get comments on the impact. Based on comments, you can either revert, move behind a flag, etc.
Isn't this a bit of a loaded take too? I'm sure the author wouldn't agree that what they wanted was for Thierry to "fold back". I agree with your criticisms here in direction but not in magnitude, in fact it appears to me like you're comitting the same sin of misrepresenting your opponent to enhance your position.
If nobody raises hell on a development branch, the change will have a way of making it to the master branch.
To add, I have issues with the attitude shown by the blog author.
If you use the development branch, you can't raise hell when there's a breaking change: it's to be expected!
Then it's fine to disagree on some change and discuss this. I read the email thread, and I do not see "arrogance". Just strong disagreement. So yes, converging will take a bit of time... Calling publicly someone "arrogant" for not folding back to your view, and trying to raise the crowd (a good part of whom won't read the thread to make their own opinion) looks like bullying to me.
Saying that his patch to make the change optional has been disregarded, when it was rejected because it not only made the change optional (that would have been OK, and a patch for this asked for) but removed other changes is not honest.
Lastly, pointing out one person to blame when the whole discussion is done with the Emacs maintainers in the loop is also a no-go in my book.
As a close to 30 years Emacs user, thank you to all its contributors! (and to Thierry, as long time Helm user) May their skin by thick, it's unfortunately sometimes needed :-P