Given that ever since introduction into service there has been a significant allocation of 450s on the Windsor lines and that these will eventually be displaced by 701s I would think there'll easily be a surplus of 450s in due course.
Perhaps so. But why on this earth would a TOC do away with units (450's) that are both newer and have a far wider/beneficial overall maintenance and operational (area and crew) regime due to number of units (127), resulting in better fleet reliability, and in 12 car format have a far wider usage availability (terminal) platform wise at (eg) both Portsmouth Harbour and Waterloo, than any older stock type (458's in particular in 12 car format) that also require additional maintenance and crew knowledge/training/upkeep/spares etc.
Without further substantial infrastructure expense (signalling and platform wise) at both Portsmouth Harbour and Waterloo, I would wager that the very limited infrastructure at those locations that can (in todays world) accept 12 car 458 formations, will be deemed unworkable in relation to any planned timetable that is required to be both reliable day in-day out, with the necessary resilience that those at the top would wish to be part of that plan.
Yes, there are (stock) leasing costs to take account of, but as above, these will only form one part of the overall considerations. I believe that those who still wish for a 2 + 2 seating on
all 1Pxx services, are still in cloud cuckoo land. The 2Pxx services (post 2004) were always planned/intended for largely 450 operation, and will likely remain so.