I can't see any other *TOC* as a long term home for the entire 220/221/222 fleet than XC (ignoring Open Access, because who knows what Open Access will be granted in future)
The internal layouts aren't suitable for most routes with 170s (given the doors etc)
There are already going to be a lot of other 90/100mph units for "Provincial" services in future (Midlands/Welsh 158s, Welsh 175s, Yorkshire 185s etc), and the seating layout/ crumple zone mean that a four/five coach Voyager/Meridian wouldn't offer much of a capacity increase over a three coach Turbostar, so they'd be wasted on something like the ex-Central Trains services
Meanwhile, the fast acceleration required to match current XC diagrams (and the tightly timed paths through bottlenecks, with little margin for error) mean that there's not a great deal else suitable for proper "Cross Country" services, unless you're going to bestow dozens of 80Xs on the franchise, but that seems optimistic.
So, for me, the question is more "when" not "if" or "should". But we all know that the railway has twists and turns - trying to second guess these things doesn't always work!