....Also if your going argue speed then really there is no need for 140mph stock to be pottering around on the ECML or WCML at 115-125mph yet both 91s and 390s do quite happily. But I get your point

....
The 225s and 390s were designed to run with tilt on upgraded mainlines at 140mph, in both cases the trains were being built or in service when the faster line speeds were not implemented (and in the case of the 225s, tilt not retro-fitted), although flashing greens on a section of the ECML are still in use AFAIK. Tilt is not currently possible with the 91s though.
....I'm sure that NXEA would rather have more modern rolling stock but I don't know of any plans to replace the 90s+Mk3s directly so I would have thought the best they can hope for is cascaded 91s+Mk4s.....
Maybe so, but would they not have to clear the 225s along the route and make sure the sets can fit in Crown Point depot (IIRC there was an issue of length with the Mk3s).
....Either way I don't see why OAO should get first dibs on current rolling stock that could find use with other franchised operators. Do you have any suggestions as to where the 91s+Mk4s could go when they are cascaded off the ECML?
Personally I think they should stay put running alongside the HST replacement, meaning new trains can be bought for other areas. The GWML will doubtless end up with brand new stock, so any move there would be short term IMO.
But, if they have to go, it's not so much a question of OAOs being given first dibs, but rather a better way to use the stock IMO, they are already route cleared for the ECML. They could be used by Hull Trains and GC with a diesel to tow them off the wires (those areas may be wired in future anyway), or maybe other OAOs will appear to use them.
The other option is to increase high speed use along the WCML. The Mk4s could, with a bit of work, be made to use tilt as originally intended, although they may need to rebuild the 91s (to create the space for tilt and keep reliability), or replace them with tilting locos, for use on the longer range LM services, freeing up relatively new 350s for other work.