Your reading skills fail badly. Mythic mode is the current heroic mode only with a fixed 20 setting. The current normal mode is renamed to hc mode and will be a flexibile setting. The current flex mode is renamed to normal mode and will also be a flex setting.
There is no added harder mode.
Regarding the issue at hand. Its a lot easier for 25 man guilds to drop 5 guys then for 10 man guilds to recruit, none denies that. While the compromise could of been at 15 instead, I don't think many realize the effect that would have on classes. Go ahead and count how many Ret Paladins/Windwalkers/Enhancement Shamans/Blood Dks killed Garrosh 10 heroic compared to Warlocks/Mages/Prot Paladins.
Lets say Blizz would add a new class in the next expansion, how insanely overpowered would it need to be to make a spot for itself?
Ultimately, nothing changes for now and guilds will only need to cross the bridge when they get there.