I was under the impression the 767 program for passenger flights was finished, and that they were only making 767 freighters. That would have been the obvious choice, as Jet2 could use their B757 rated pilots to fly these as their share a lot of traits.
With the new 738's coming in, i would assume Jet2 are moving away from the 2nd hand market. But it raises a good point, the B752's won't last forever and a few years down the line, they'd need to consider alternatives