This could be useful if a PR is essentially changing only one thing like a commit should, but since PRs usually cover a feature or a fix for a ticket, these frequently involve doing multiple things to implement the feature or fix the bug.
> the [option of enforcing the quality of commits in a PR] does not provide any practical benefits for the process (rather than theoretical “but we’ll have individual commits when 5 years later someone blames them”),
Your parenthetical clause is not theoretical. It's basically a thing I have to deal with on a daily basis when I'm fixing bugs or removing/adding features in multiple code bases that were written by people years ago who no longer work there. I can't really make use of git blame because the commit messages are non-informative, the diffs are just work in progress saves, and the linked PRs just have looks good to me comments and no description.
But for code bases where commit quality was enforced, git blame actually becomes useful and allows me to see what was done and why it was done at the time it was written.