The A320NEO does have 189 passengers in a one class layout, so that’s one objection solved.
If we are increasing capacity, and buying new aircraft, does a logical solution not lie in the 737MAX-8,-9 or especially the 737MAX 200 - an extra 11 seats, and more economic - less fuel consumption, similar parts library...
But anyway, new aircraft are unlikely to Jet2. Maybe when the 737MAX comes out, and the 737NG prices fall, then Jet2 might consider buying new then. But lets hope a new aircraft order comes sooner rather than later.