From what I see Jet2 doesn't over work it's classic fleet and they seem to have built plenty of contingency plans in to their fleet structure. But age does matter and no amount of new paint & Recaro seats gets around this in terms of reliability, put simply there will be more delays, diversions with older aircraft than newer established aircraft ie the 738 not the 787 !!
The classic is still a very worth while tool on standard med routes it's ideal on AGP/ALC from the UK and the lease price is so low now that it offsets it's much higher fuel burn per seat.
Whether Jet2 can keep them going until 2022 is another matter, of course the 757 are older still but still look the part, the average punter neither knows or cares, jet2 is not a network carrier in the way that FR EZY or NAS so a delay whilst frustrating is not terminal for their customer base which is seasonal buck n spade
I hear very few complaints about them and when it goes wrong they spend the money saved to sort it out.
With regard to US carriers running old fleets this is true, but they have a lot of spare capacity so the impact on punters is less
VC10 now that would be a fun flight, not profitable but fun all the same, LBA at 2am would get plenty of publicity lol