As a creator, it is sometimes advantageous for me to deny some of my own work. I regularly use "fixup" commits and rebase them out of existence. I'd be mildly embarrassed to have all of them sticking around in code forever.
As a user of code published on Github, I sure as heck want there to be clear ownership and authorship of the code I use, and to be able to hold them accountable for malice. And there is lots malicious code on Github, trying to disguise itself as legit; Having a strong public persona isn't guarantee that my github code isn't malicious, but it does raise the stakes.
As a human who has seen what happens when people commit mistakes, we're very emotionally bad at dealing with mistakes. "Who broke the vase?" is a tough question to ask. "Who broke the build" can carry the same weight - Both very little, and very much, depending on when it's asked and how expensive the vase was. I'd like to think that making mistakes will be easier in a merkle-tree structured world; That as a collective, we will learn to do a better job of helping people admit their mistakes, come forward with them, and say "I made that mistake, and have learned from it". The internet has proven that false again and again.
In two important ways - People are unwilling to forgive, and people are unwilling to admit. Past minor mistakes haunt people forever all the time. Look at things like "Has Justine Landed Yet?"[1].
A malicious liar (We'll call them Eve) will deny, refute, and plain old lie in the face of whatever. There can be incredibly convincing evidence (A RSA-Key Verified Git Signature, for example) of... whatever, and it will be written off as "Harassment" by friends of Eve. Further: There's plenty of actual malice that passes itself off as not. Rapists say "She clearly wanted it, she got wet" - Criminals do that at all levels, making up excuses for why their victims deserved it.
Regardless - It is a sea change to be held accountable for your code. I do want it, but I do not believe that SSH key verification will achieve that. I think it will be a significant change when it does happen, though, and one with significant consequences.
[1]https://www.nytimes.com/2015/02/15/magazine/how-one-stupid-t...