This argument hasn't changed in twenty years, in spite of massive improvements in ease of use. Apparently, it's impossible to make it "easy enough" for the average user. I think this means ease of use actually has very little to do with the problem. The problem is with the average user's priorities. People value convenience more highly than privacy, and as long as people don't change those values, encryption will never take on.
Typically people will only change their priorities under threat of dire and immediate consequences for them personally. Everyone will lock their door so they don't get burglarised. But email privacy is too abstract and invisible still. It's going to take some huge cases of identity theft, with real monetary loss, to get people to change â" and then people will probably sooner abandon email than use email encryption.
Finally, the kind of convenience that you propose necessarily will render the whole thing insecure. Letting strangers (like Google) manage your private keys defeats the whole purpose.