The economics of operating 1x 325+ seat aircraft doing 2 short haul flights a day on high demand routes with 2 sets of crew must work out better than utilising 2x 189 doing 2 flights per day with 4 sets of crews. As a holiday company you also need to look at whats going on in resort.
I'm with you on this one. It definately makes sense with them trying their hand with the leased A332, Some of the largest bases/destinations are more slot constrained, either by terminal size or runway/parking space. certainly from a runway/ parking perspective, to Remedy this, consolidating 2x 189 on to a widebody frees up a spare slot for future growth, while maintaining the same capacity on the existing routemakes sense as you deliver the same
I only suggested the B788 since Jet2 like to own Boeing and lease Airbus. An all economy B788 would probably get in the region of 350 seats, close to 2 x B738's at 378 seats, that was the logic behind all this