Hopelessly romantic "wouldn't it be nice" extension and "public service" proposals are, IMHO two of the bigger threats to the continued existence of preserved railways. It is a lesson that each generation seems to have to learn, painfully, over and over again.
Public service proposals should take priority over the desires of the owners of preserved railways to "play trains", provided the business case adds up.
You also have to remember that most Heritage lines are run by people who want a full sized trainset rather than people who want to make a lot of money (yes there's people who are commercially minded, but only so far as to ensure that the line continues to be able to run).
It is likely that heritage lines could make more money, at a lower risk, by buying and relocating signal boxes for people to be able to stay in for 3/4/7 day blocks.
The state, in the form of the local/regional/national authority as appropriate, should have powers to force heritage railways to provide public services in liaison with local transport providers, if the proposal is deemed to be financially viable and in the overall public interest after careful evaluation.
For the example of a Rawtenstall-Manchester Victoria service, I would envisage it being under the control of TfGM/TfN and run by Northern Rail with modern access-compatible dmus in liaison with the ELR. There may need to be some publicly funded track/signalling upgrades to enable higher speeds and adequate capacity (?dynamic loops) between Heywood and Rawtenstall. The service I would envisage is as follows:
Mon-Fri 0700-1000 and 1600-1900 every 30 minutes Rawtenstall-Victoria, run by NR; no ELR services at these times.
Mon-Fri 1000-1600 and Sat 0800-1800 every hour Rawtenstall-Victoria, run by NR, with an up to hourly heritage service (frequency varying according to season) run by ELR between Rawtenstall and Heywood.
Suns & Bank Holidays - no NR service, ELR free to run whatever trains they wished between Rawtenstall and Heywood.