Your making a lot of assumptions there, the GC HST's may well be only temporary up the 2020 compliance date, new Bi-modes for the MML are clearly not going to be around by 2020 so some sort of decision will have to made as what happens with EMT HST's beyond then it could of course include swapping MML HST sets for VTEC and/or remaining GWR ones.
The only way bi-modes could possibly make sense on the MML is if the plan were for staged extensions to modernisation until the route is completely modern. Since the current plan is to let the line rot until at least after HS2 Phase 2 is built, there's no point.
The only reason bi-modes have even been proposed is to keep diesels out of London. Londoners matter to the government, people in the Midlands/North do not. I'm not sure why, London voted predominantly Labour, as did Leicester, Derby, Nottingham and Sheffield. I suppose it's the fact that they've got a mayor...
Even if the MML did get bi-modes, nobody's yet given a sensible answer as to where the 222s are expected to go. A silly microfleet of a few bi-modes alongside them makes even less sense.
The idea of swapping unrefurbished carriages for ex-VTEC carriages once they become available would be nice, but the D(a)fT would never do anything that makes that much sense. I'd be surprised if chief idiot Grayling could even find Nottingham on a map, let alone know the differences between different fleets of Mk3s. Even if it did happen, the East Midlands would probably end up with ex-GWR sardine cans out of pure spite.