Going loco - the choice MAN's current loco's are offering (particularly on the city destinations) seems to be shrinking all the time. So to be honest although a possible Easyjet base wouldn't be welcomed by all, there would hopefully be some important destinations re-appearing on MAN's departure boards.
I take you're point on fares though, if they do indeed set up at MAN and see some of the other loco's off certain routes, you could well see higher prices on route where just they end up operating.