You asked the rhetorical question and then I answered it in my head before reading the rest of your post. When I read the rest of your post, it was exactly what I answered in my head, down to the Keaton example.
I’d also venture the Marx Brothers made more money and didn’t have to work as much as The Three Stooges - but then I thought of Chico’s gambling problem.
From what I gather, Groucho was wise with money, and lived very comfortably during his movie career and right to the end of his life. I think I read somewhere that the rest of the brothers had to give Chico an "allowance" so he could at least have some sort of roof over his head and could eat, but he of course still gambled most of it away.
I assume Harpo was careful with money as well, since I've never heard otherwise.
Zeppo complained in THE...SCRAPBOOK about Groucho still working in his one-man show in the 70s, questioning why since he didn't need the money.
In that same interview, Zeppo pats himself on the back and brags about how successful his agency was, and seemed happy that he got out of acting. He said there was no animosity with his brothers when he left -- he claims he was pushed into being in the act by his mother when Gummo was drafted into the army. He was of course much younger than his brothers. He still comes across as a self-aggrandizing snob in the interview, much like Jerry Lewis!!! He makes a point that with his brothers, he was told what to do, but with his uber-succeful agency, he told people what to do! I'm sure Groucho probably told him where he could go, and where he could put his agency!!!
So yes, The Marx Bros. made great money and didn't have to be as prolific as The Stooges. But we as fans would have been better off had they made much more, and I for one would love to have 190 Marx shorts to view!!!