From Australia, in a few months I'll hopefully be travelling on five or six UK heritage railways in 14 days before we head to Europe, plus fit in one Railway Touring Company day trip. Previously I've been on a small number of these heritage lines.
The one I couldn't fit in was Llangollen, because it lacks a main line connection (although bus frequencies seem quite good from Ruabon).
WSR railcams suggest patronage is highly variable. On better weather days, excellent. When it's slightly foggy or rainy, much worse. This line may eventually have semi-regular trains, probably DRCs, to and from Taunton (perhaps on selected weekends?) to link at Bishops Lydeard. So despite having been in financial trouble during COVID, it has a future. It's well promoted with official and unofficial great websites.
Those who state 'there are too many' may omit that you have a population on a relatively small island of 67 million. In comparison, my state of Victoria in Oz has a major 762mm narrow gauge tourist railway in Melbourne (Puffing Billy) and about five other heritage lines, the most faithful of which is that operated by Castlemaine and Maldon Railway Preservation Society, the Victorian Goldfields Railway. Our population is 6.6 million in a state that covers an area about 80 per cent of England, so population density is low once out of Melbourne.
Why don't some of the larger heritage railways like GCR, NYMR, SVR and WSR try to attract more international tourists? Sure, Japanese and mainland Chinese can't come to UK at present in numbers, but these nations plus India has many who love railways. They'll only ever be a minor source of income compared with UK locals, though. Citizens of these three nations are not like USA residents who typically are not used to travelling by rail.