Except when you say to someone, I'm going to keep x% and you're going to get y% (where y is much less than x) because your future contribution isn't going to be

Except when you say to someone, I'm going to keep x% and you're going to get y% (where y is much less than x) because your future contribution isn't going to be as much as mine, that person is either a) going to walk away or b) have no motivation to perform at startup levels because you just screwed them over, especially if you're not out there killing yourself every day.