The difference between Parker and the other three rotation guards is, unlike them, who are all in ideal roles, he's now miscast in the one he's in, as lead creator for a team with championship aspirations. Even if Leonard assumes that role, he'll still be miscast in more of a spot up capacity.
Ginobili is better and was always going to age better for reasons that have been discussed ad nauseam, but he also gets to play about 8 less mpg, mostly against backups, at a lesser position.
It's not exactly breaking news, but the takeaway from this is that all four rotation guards are relatively close and none are close to elite and that's primarily why this team isn't a championship contender anymore.