The thing is they've already tried getting a new Master and look, Gildarts ran and Makao sucked. Makarov was pretty much the only one to take the job, and rightfully so - the Guild is full of insane juggernauts. I don't think Erza has the leadership skills anyway, real talk. But I feel like the Guild's gonna honestly disband after this war.
Spoiler:I think Mashima's gonna pull the rug from us this time and have Eileen be part of a clone project or something instead of future Erza or whatever.