That wouldn't be a problem with genuine flighting, i.e. the West Mids train comes up behind/in front of the Trent Valley train and does the intermediate stops that the TV train misses out. The TV train could then go fast Preston-Carlisle, with the TV train doing the stops. That sort of used to happen but the capacity problems with the single 221s put an end to it.
Ah, so a bit like the LM service on the south WCML, which as it is robust and reliable is heavily used by commuters from smaller stations further north? The best example, and one used by well over 50 people, is taking the 1816 to LBZ then switching to a slower service for Bletchley.
But for it to work those connections have to be robust. In the days before the Euston-Brum being joined with the Brum-Scotland, so MKC passengers sort-of used the service in that way (take 1840 MKC-Crewe, then change for Scotland) it was stressful, and southbound the Sunday evening connection was something ludicrous like 58 minutes.
FWIW, the LM Euston-Crewe service used to work like that at Stafford, sort-of - the connections to/from the Liverpools for a faster journey to Crewe were excellent and same-platform. Post Project 110, they no longer work. Though that isn't the primary purpose of that service, so it is understandable.
So overall, I wouldn't be *strongly* opposed to that approach provided the local service has capacity, and provided connections are (a) reasonably short, ideally "perfect"[1], and (b) guaranteed to any reasonable extent (i.e. held in the event of disruption). I find it hard to believe that the UK system would have the will to achieve that, and as such I find it hard to support it.
There is also the catering downside - I actually quite like putting a 30ish minute connection into a very long journey so I have time to obtain catering given the reduction in provision on board. I would of course prefer to be able to take a restaurant car meal and stay on board, but such is progress
[1] The two trains come in either side of the same island for interchange, and remain there for long enough for this to be relaxed and not stressful, and the trains are held for a reasonable amount of time if the connection would not be achieved.