Jet2 @ LPL realistically has nothing to do with Manchester.
They have been taking A321s and second hand 737s by the ton since the start of the year. The LPL base will be a surplus of new aircraft, not aircraft taken out of MAN. Further growth at MAN will largely be driven by the swapping of based 737s to A321s.
Liverpool for Jet2 is mostly a new market. As has been stated all destinations to be offered ex LPL are available in much higher frequency ex MAN.
The base will compliment MAN not take away from it. Look at easyJet for example.