XC's problem is basically politics. Any 'London to...' operator' will have MPs pushing for plenty of capacity on the basis plenty of them and their staff will use it. Any 'local' operator can get local politicians championing more capacity. TPE may cover a wide area, but being northern, centred on Manchester and providing some decidedly local services will get the Greater Manchester 'establishment' behind it.
XC on the other hand doesn't serve London and isn't a local operator to anywhere. It is a vital part of commuter operations into Birmingham in particular, as well as other cities such as Bristol and Leeds, but isn't perceived to be 'their' operator by anyone.
“The people” at XC would have liked them but they had to fight hard with the DfT to get what they have got.
I seem to recall that within the XC consultation document around 2016 or 2017 proposing to cut services at the periphery so that capacity could be increased through the core, the then head of XC did state that most of their capacity problems could be resolved with a handful of additional units. The tone was sheer exasperation that they couldn't fix this issue.
The conviction that XC management are perfectly happy with the status quo doesn't add up. Their staff must be fed up with working packed out trains and receiving abuse from customers as if it's their fault, when nearly all of XC's problems would fall away with improved capacity. It must be a major morale issue. Assuming this is fed back to management, they'd surely want to do something about it.
Anyway, it's good that XC have acquired some additional capacity, even if it's not as much as it could be. I do hope that they stop running 221s in multiple though - it seems ridiculous running a 9 coach formation one hour and a 4 coach formation the next on the same route. Even dafter when the 4 coach formation offers better advance fares (something I've noticed comparing the usually 9 coach 1645 southbound out of Bristol with the frequently 4 coach 1745). 8 and 5 would clearly be a better spread of capacity.