Until HS2 comes along, they should be kept on the WCML for obvious reasons, but the question is what to do with them after. After that point though, I don’t see what use they would have.
Of the London bound WCML services, it would make sense for Birmingham and Liverpool to continue with a 350 operation. Therefore given that model works, it also makes sense for any Manchester-Euston WCML services to go to something similar to a 350. At least a train with doors at 1/3s. They won’t be needed in Glasgow-Edinburgh either.
They will be about 35 years old at this point in time, therefore scrapping them may be the best option, along with the Voyagers. (Although I fear that Voyagers May become the Pacer of long-distance travel).