Mob cap isn't based on how many players are loading a single mob-capped chunk. This is related to MC-28346.

A chunk based mob cap would mean the amount of players loading a same chunk would multiply its mob cap, thus fixing/avoiding the "first player gets 1.5 times normal mob count, second player gets 0.5 times normal mob count" quoted from MC-28346. This should nerf mob farms, and make game difficulty server-wide instead of the current way that I can't find the right words to describe.