Its as if a tennis player were to win Wimbledon, the US, French and Australian opens, and is ranked 10th on a list of players who won a bunch of bank or super market promoted events.
It's like... nothing at all like that.
Ignoring the fact that tennis isn't a team sport, where the richest team can buy you the best helpers and fundamentally increase your chances of winning. Ignoring the fact that tennis is a succession of 1 v 1 games where two opponents face each other under exactly the same circumstances. And ignoring the fact that a grand slam differs in virtually nothing from any other tournament, other than the number of sets for men (not women) and the amount of rounds that are played (due to more contenders).
In tennis, every player will try to participate in every grand slam, unless he is injured. Never have i heard a player say, that he would skip Paris and London, in order to focus on the US open. In tennis, there is not one player who will play only one grand slam, and a few prep tournaments. If physically possible (getting older plays a part as well), they all play all the big tournaments, and a bunch of smaller ones in between, depending on which suit their schedule best.
And the biggest issue with your comparison, is that any tennis player that can win all 4 grand slams (where the usual problem is that most don't play equally well on each surface, clay, hardcourt, grass... bringing your comparison closer to cyclocross vs road vs MTB), any player that can win all the slams, can win ANY other tournament. Which is exactly the problem of the entire discussion here. Froome can't win Flanders, Froome can't win Paris Roubaix, Froome can't win E3, Froome can't win Strade Bianche or San Remo, nor anything ending in a sprint. Him winning TDF, doesn't change that. That's just the entire issue with the debate, that a guy like Sagan wins stuff Froome can't win, and the other way around. And which to value more.