Surely the timescales aren't going to be right for that? The GW» electrification will be done before MML» won't it, and bi-modes will be needed for the Cotswolds, Cheltenham etc services before any displacement of 22xs could take place.
New trains will be needed when
HSTs▸ break, rather than when the wires go up; there's no reason you can't keep HSTs running under the wires.
FGW▸ have seemed fairly optimistic about the chances of keeping them going for a few more years, and obviously electrification to Bristol and Swansea reduces the size of the fleet that has to be maintained.