I'm not sure which bases the 733 currently operates at, but as a general view of overall capacity, when the 757's are retired there will be a loss of capacity from the 757 to 738 of 46 seats whereas to replace a 737-300 with a 737-800 is an increase of 41 seats so there's a fairly even balance in replacing both of those ageing types. Having a standard fleet will also save costs
B733's are now only based at BFS/EMA and LBA. B752's are only at LBA and MAN, all the rest are B738's. Hope that clarifies that issue.
People don't realise there are still a few relatively young 733's. G-GDFG/K/L/N/O/T are all between 20 and 22 years old, the way Jet2 treat them, they'll be around for a good 8-10 years yet, A couple of these are even younger than the oldest B738's in the fleet (G-GDFD / P)
Yes the majority of the fleet will be B738, but there will be several routes which just can't sustain the 189 seats on offer and actually makes it better to operate the 148 seats on a B733. It also gives then the option to switch capacity where it's required.
On the flipside, half of the B752 are 31 years+, so these must surely be on the retirement radar. Jet2 have dabbled in the A321 and A330's looking at the extra capacity they offer, and i've no doubt they could sell a lot more on some of their most popular routes, Ideally we'd need to see some sort of deal being made this year to allow these to come in and the B752's to phase out. This discussion has come up before on this thread, so wewon't dig any deeper than we need to.