My thinking is that you're not duplicating cabs wasting space that could be used for passengers, an 8 coach Voyager has 4 cabs, probably more than an entire coach
The voyagers have large cabs because they are 125mph stock while the only meaningful 125mph running is the ECML and Birmingham - Derby. Manchester - Bournemouth and Bristol have no 125mph running aside from a tiny stretch just outside Bristol. A class 196, 350, or 444 wastes much less.
lots of duplicated equipment e.g. compressors
You want duplicated compressors, if the main one fails the train will need to be taken out of service fairly urgently.
You also only need one set of staff either loco hauled stock not 2 sets if two units coupled.
Not on XC, a single catering cart wouldn't get through an entire 8-9 car train anyway. The Pendolinos typically have a driver, guard, 2 first-class catering staff, someone on the shop, and sometimes an onboard chef.
I would expect reasonable energy savings with a loco and 8 coaches over an 8 coach unit, especially in diesel mode.
I wouldn't, with less driven axles you get less effective regen. You also have the weight less distributed so worse track wear which NR punishes with higher access charges.
Performance is largely irrelevant as already been said that journey times have hardly improved over past 40 years. Modern 3-phase drives will give a loco reasonable performance when up against a unit.
But they need to get near voyager timings in the busy electrified sections to fit in existing paths. A class 68 with a few mk3s or mk5s is nowhere near a voyager performance (closer to class 168). For a long set of mk5s they'd be needing 2 68s.
Traction motors aren't the issue, you're limited by how big of a diesel engine you can squeeze into the locomotive.
Passenger space seems to be the important thing here and for that loco and stock makes sense. If need extra coaches in sets don't need extra staff.
The mk5s aren't that space-efficient and no manufacturer is going to be rushing for a small order of purpose-built coaches for XC.
It would be absolutely fine with modern bi-mode EMUs, say 807s, which are 6-7 cars minimum
Even at 342 seats, a 5-car Class 802/2 would approach the capacity of a double 220.
Single 7 car would probably be about perfect.
Yes, the multiple-unit operation at XC is only because of the poor capacity of voyagers. On a completely new fleet, you wouldn't bother and would go with a standard fleet of 5 or 7-car 802-style trains.