It makes absolutely no sense to suggest an increase to 125mph on the SWML is a better solution than running bimodes on the 3rd rail.
a) If that was a viable option it would have been looked at long ago with the advent of the HSTs and Voyagers on the SWML, which have been running for around 40 years at this point. It isn't for two reasons: firstly, because it's expensive for only a few trains, and more importantly because of the capacity constraints of trying to run 125mph trains along that route without also increasing the speed of the large quantity of commuter services that also run down to Southampton and Bournemouth. It's completely unrealistic to expect them to be able to eat up all those paths.
b) If they can go current linespeed, it would save money for them to have a 3rd rail shoe so they can run electrically, rather than running diesel over the rails with no performance benefit. Indeed, it would be cheaper in addition to providing a better ride experience. Otherwise your argument that they could "just fire up the MTUs" is an argument against all electrification full-stop: ask yourself why we bother to electrify in the first place! This is ignoring the possibility of an increased XC network, which is something they're looking at in the longer-term, and will become far more possible as HS2 begins to relieve capacity. Running down to Brighton etc. used to be a regular occurence. Even without this speculation it would still be worth it if XC SETs were ordered before 2020, as you won't see any SWML overheads until at least 2025.
Finally, your point about the Voyagers having a lot of life left, there are plenty of places they could be moved to. Replacing the Scottish HSTs for a start, which will almost certainly have to happen at some point. And frankly, it's only enthusiasts who seem hellbent on running trains to their maximum service life: the DfT and NR are far more interested in saving themselves money and getting good PR. British "built" trains, a more uniform long-distance fleet and more electric running under wires and (possibly) 3rd rail would be good for both of those.