I suspect that XC (or its successors) will still be using 220s & 221s on most of their current routes until they are worn out, so possibly until at least 2035 /2040...., when they might just get some of the 2040s equivalents of Class 800 (but probably no longer than 5 coaches.)
I 100% agree with you about the Voyagers. I can't see XC getting any extra trains until at least the franchise is renewed. There has been an extension to Arriva's contract due to the DFT Government and public consultation (which I participated in) and also very little interest shown by other operators in the XC operation. Remember that XC (as do Virgin) use the dedicated Central Rivers depot for major upkeep of the 220/221 fleet. The fleets builders (Bombardier) also maintain them in this dedicated and bespoke site, so shifting them to other operators away from this dedicated base in the Midlands would not be a favoured option. Voyagers are very reliable considering the milage they do and that is mainly down to the manufacturers dedicated and bespoke maintenance facilities at Central Rivers. The true enthusiast (myself included) would absolutely love to see cascaded GWR HST's on the NE/SW corridor to supplement XC's current 5 sets, but that is very unlikely to happen for many reasons:-
1) Higher track access charges for XC to use a HST instead of a Voyager.
2) Corrosion work, Toilet Tanks and new doors would have to be fitted to every MKIII coach and Wabtec is light years behind delivering to Scotrail/XC/GWR already.
3) Each MKIII coach would have to have an interior refresh plus work on it again to ensure it could travel over 3rd rail territory from Basingstoke to Bournemouth (If it was ever planned to use them down there?)
All the above mentioned work (at the current rate) would take years, by which time the franchise would have probably been renewed and the new operator would be looking at new stock to phase out the Voyagers (a bi-mode Class 802 for example, who knows?). Either way, I think we are all stuck with Voyagers on the XC franchise for the foreseeable future. However, XC HST's will be moving from Edinburgh Craigentinny to Plymouth Laira for base maintenance because Craigentinny will be gearing up for Azuma maintenance and Scotrail HST's will be looked after by Haymarket.