The problem is you guys are very grossly overstimating the contents of the Great vault.
It’s 9 possible choices if you do 10 raid bosses, 10 keys, and an ungodly amount of PvP. And the raid and PvP choices are capped at your top level : you’ll get heroic 213s unless you mythic raid and your PvP choices are based on rating.
So realistically, a more casual M+ runner won’t even get the 9 choices, he’ll get at most 2-3 on his Mythic+ row and the rest will be much lower ilvl.