Another thought is given that there's never been a requirement for all 127 450s on the mainline (there has always been a significant allocation on the Windsor lines) use some of these "surplus units", eg on the through Camberley line peak services for instance. Then SWR can get shot of the 458s as well.
Thinking from an SWR (ie present situation) you may well be correct re 450's, but 120 from 127 was in past times the maximum that could ever be used (by contract) on a day by day basis, and this did drop now and again (during SWT days) so that specific mods could be undertaken, without interfering with the seven units on routine maintenance etc., eg conversion to 450/5, and then return to standard format (with 1st class) during the original 10 car introduction (458/5 etc), fitting of new dampers fleet wide, etc etc. Those initiatives can't be undertaken by robbing the maintenance guys of their bread and butter for months on end, otherwise the train service would start to suffer sooner than many would realise.
Think I've said before, but the service level as it was pre covid, was, and would again, if reverted to a similar context/format (and not enhanced as per SWR's bid) still be coverable, EMU wise, by 444/450 and 701 alone.
But, again as I have also mentioned elsewhere, things have changed, and patronage may be a tad different these days, not to mention the issue of 158/159 approaching their twilight years and all that that may entail, so things are different now. That said, I believe the first initiative in all of this must be to get the entire 701 fleet (as ordered) up and running to cover the entire suburban service group (Readings included - sorry) , but as intended. Until that is accomplished, there may still be doubt as to what can be achieved with the remining fleets - eg: how many 444/450 will actually be needed for the wider main line service group? Much of the future may well still be based on assumptions.
Therefore how long do the 458's stay on the books - or will they be required to replace 450's off loaded to this suggested WoE line battery initiative? If this latter scheme is a serious contender, it would make more sense (number of units, bespoke crew/maintenance knowledge) for the 458's to replace the 158/159 fleet perhaps? The numbers don't quite add up I admit, so some form of timetable revision would be required, and also that old chestnut of 12 car 458 still being too long for most platforms at Waterloo as things stand.
Interesting times indeed.