There is nothing wrong with the interiors of Jet2 fleet based on my own experience as a passenger, they seem to have created extra legroom with the new style seats which despite being thin are ok for a couple of hours, a few seem to have dispensed with one of the rear toilets which is a novel idea.
The policy of buying old seems to work for them, but its not all sweetness and light, they have to have back up aircraft dotted around, with crews, ditto engineering, but on the whole it seems to work for their target market, which is bucket and spades and city breaks, a market that tends to be forgiving of delays and turn backs in a way that high frequency city pairs wouldn't be, it all about knowing your market and your customers. Good on them