A321
I think you have your answer, when it becomes clear that it cost more to operate old 737's then it will be time. The only other 737 loco with elderly 737 is baby and theirs work flat out from dawn until dusk, they aren't as old but baby seem to get more tech delays than Jet2 probably cos they work them harder.
What may alter the cost/benefit equation is the planned changes to APD in this months budget if as reported they move towards a per aircraft departure charge rather than per passenger and if its based on ETS emissions, then a cleaner aircraft may have a cost advantage (which is what Easyjet have lobbied for and i agree)
This would be a blow for Jet2 because it would lower the value of their fleet and increase costs, they also do a fair bit of ferry flights on the flip side it would reduce the purchase price of an additional aircraft.
The likes of baby with a leased fleet would be stuck with the additional cost burden, a least until lease expiry.
I don't agree that pax don't care are old the aircraft is, its just that they have no idea, i am not sure they would be to happy with " Laddies & gents welcome aboard this 23 year year old Boeing 737 flight to Malaga"
I am rather surprised that Ryanair have never picked up on this with their fuselage adds " the baby on the right will be on average 14 years older than this aircraft the Jet2 on your left will be on average 20 years older
Still one could retort that the f/o on your aircraft is being paid for this flight whilst the F/o on the Ryanair on your left is paying to do that flight!