Also, a new train type could provide XC with a unique selling point, and hopefully improve many issues faced. As much as I love 22x units, when the time comes that they get life expired, they should be completely replaced en masse.
Given that we're already talking about a fleet of 68 units, most likely to be 88 (with the addition of the Virgin 222's) so at the very least 340 coaches, but most likely 440+ coaches. This compares with circa 600 coaches which is what GWR have.
This is likely to mean that any delivery is going to take a bit of time (probably not as long as GWR's fleet, but likely to be at least a year).
It's already going to be 2020 before any decision is made on the running of the tender for a new franchise. From there to start of delivery is easily 3 years, which would put the 22x fleet at about 20 years old.
However it wouldn't take much of a delay for them to be reaching 25 years old (especially if you add the 222 fleet into the mix so that the franchise doesn't have such a strong need to be relet for a few more years). If you then assume that it's likely that they won't find a new home for a year or two after coming out of service, then it could well be that you're not going to get much more that 10 years of service or of them and even if something were to happen this week you'd be looking at but much more than 15 years of service for anyone else.
Add in their high costs, the desire to decarbonise the system (which should see electrification of much of the XC network), and their poor capacity when run in short lengths (they wouldn't be too bad if they are 7+ coach unit lengths) and you could see the logic in scraping some of the fleet early.
I do however wonder if you could create a very powerful 4 coach EMU which you could sit between 2 halves of a Voyager which would enable you to run 8 coach trains with bimodal capability (probably with 90mph running off the wires). By doing so you could just scrap the Voyager bits and still be left with a EMU that would be used beyond the end of life of the Voyagers. You may struggle to get the acceleration of the Voyagers when running only on electricity, however this could be supplemented with some Diesel power to boost performance.
You'd probably have to rewrite the control systems so there's going to be some fairly costly changes, however probably still less than building 2 extra coaches (assuming that you're getting an 8 coach train for 15 years and a 4 coach EMU for a further 25 years and therefore the 4 coaches of the Voyager would be used for nearly half the life of 2 coaches).
Although that could leave you with need to buy extra stock in 15 years time, depending on where it was being used that could be just a straight EMU, or cascading of 80x's with them being replaced by EMU's.