I've never been to Bolton, and can count the trips I've had to Manchester on one hand, so not my area of expertise, but I would add that I think any service to London from somewhere that currently has no service there would always gain usage. It might take some time, but if run with one in the morning rush and one in the evening rush I think it would become attractive. Especially on a route Virgin sign for ECS. What would work well I think (again, just sketching ideas from observations, more than happy to be proved wrong) is a ten car voyager that splits/joins at Piccadilly. But I know platform space is like gold dust there so it can't happen.
I also agree on ticketing. With that distance I'm sure many people split in Manchester, particularly those with cheap advances London-Manchester. Therefore it would make sense not seeing many London-Bolton through tickets.
Also, again, I may be wrong, but don't TPE provide the faster Manchester/Bolton service? If so wouldn't most Bolton/London tickets be on TPE and not on Moonshots northern train?