Many guilds have the problem that they cannot find players who commit to being there every week and completing the 20 that are needed.
Recucing the numers of player, takes away that pressure of having to be recruiting and only people who just want to go for a few days and then leave.
In my opinion reducing it to 15 for exapmle, would already give a lot of life to medium guilds that cannot doing Mythic raid when the patch is already advanced due to lack of people playing the game.
A good implementation would be that when the first 200-300 guilds have attached 10/ 10MM and the Hall of Fame is full, put a hotfix for the rest that we do not care about the world first and let us continue mythical proges.