Whilst I agree that one or two smaller operations may fail, I doubt that there will be a major HR failure. I think there is now a far better understanding of the various funding methods and benefits of HRs to an area so support is obtainable from a range of sources.
Obviously there is also the question of reduced numbers of volunteers, but I suspect that in part is due to the success of Heritage railways. There are now more railways open, for longer hours than ever before (Although Covid has had a significant effect, as has coal price/availability). Thus, whilst there may be fewer volunteers per railway, overall, the numbers of volunteers may actually be greater - also the number of active over 60's is increasing as population demographics show.
It must be significant that there are several railways that are still expanding, despite the difficulties, so the sector is healthier than some on here seem to think.
Yes I am an optimist - but I can recall reading that there were too many lines opening and there would be failures......back in the 80's, 90's and so on!