They just expand by bringing flights in from other airports
Ryanair got over 70 bases around Europe so that could be 70 flights per day off peak. Without using Manchester based planes.
Same will go to the rest of the airlines that have bases else where.
Thomson could do for example
NCL-PMI-MAN-PMI-NCL.
You may find that the airport does ask the likes of thomsons and others to move aircraft to offer these kind of flights at a much less rate.
Very risky though