'Whilst other airlines come and go' - I'm surprised that flybe have such a short memory. They have shot themselves in the foot if they are referring to LPL/IOM flights as flybe are indeed an airline that came and went from LPL leaving a rather sour taste in the mouth. Pax have deserted the LPL/IOM route in droves since they took over as Pax were disappointed with them and the route is stagnant.
Its a pity really as when I used to fly LPL/SOU and LPL/GLA the airline was actually okay, although unreliable in relation to timekeeping and the flights were poorly timed, but once onboard the product was good for these short haul routes. The suspicion was that the routes were only temporary as a bargaining chip to entering Manchester at good rates, which they managed to achieve, so the advertising blurb, as I mentioned earlier, is incorrect and poorly timed.
After stating that, IF ezy compete on the route, I doubt that it would be more than once daily and there is more than sufficient traffic for both airlines to prosper, without even impacting upon the ferry. I would surmise that Flybe would retain all if not most of the business traffic, plus medical transfers and EZY would attract more holidaymakers in both directions, particularly as they have a much higher profile in the Liverpool area. EZY have 'grown' many routes and this is one that is patently underserved, so it shouldn't take much.
I dont think that this would have any more than a minor impact on other routes from IOM. I can see both airlines on the route for a few years at least, so I dont believe that flybe needs to do anything in particular to maintain a profitable market share.