I'm going to say...yeah, it was necessary. In fact, it changes everything.
I think an exiled Krang would mean an entirely different storyline, especially if the "Oncoming Doom" (which for some reason, reminds me of "The Oncoming Storm" moniker for The Doctor by the Daleks
) is going to have significant implications for the story onward. It would mean a significantly calmer Ch'rell, which would probably be the factor that would decide how drastic things might get.
An actual "Krang in exile" would still be a fun storyline, but that's for an alternate universe where Kleve and Montuoro got Krang out of Leatherhead's path.
The turtle probably know
less than we do about the Triceratons, and that's probably going to be difficult for Donatello when he has to be a mediator for them and the Utroms.