Indeed and even the occasional hikers are those prepared to pay a very high fare per kilometre compared a normal rail fare - most do probably because the fun ride adds to the experience. Even in the few loactions where a heritage line could generate significant 'normal' traffic, they tend not to as they run only seasonal sometimes weekend only services with a price set to the norm for a 'day out' rather than the fare from A to B. It is very hard to devise a charging / fares model that keeps the premium paid by tourist traffic, while allowing 'normal' passengers a level of value for money comparable to other public transport. There are a few places abroad that do mix heritage operations and 'normal' public transport, some of the narrow gauge routes in Switzerland and a few of Japan's 'third sector' lines being examples.
In the UK, even where the line has been are minded to try to provide such a service, the costs and difficulties tend to be out of all proportion to the traffic available, witness the Swanage railway's failure to meet its long term aim of permenantly linking services to Wareham, despite being parrallel to a busy road and bus route.
At first sight the WHR might seem to fit into this category, being the only rail service to Caernarfon, but to travel to Cardiff, Birmingham or Euston by rail via the WHR and the Cambrian would likely add four to five hours to such a journey compared to driving to Bangor, even if they had a few DMU one person operated 'normal traffic' services in addition to their very intensively staffed heritage services. Instead the Welsh Highland seems to be focused on being a mini 'Glacier Express' like experience. The Harbourmaster train from Porthmadog for example carried at least six members of on board customer service staff (and I assume most of these were staff rather than volunteers) and includes an element of on train catering (bookable). A shame they don't use the Glacier Express, with their panoramic windows as a model for their coaches though...