Whilst I agree more could be done to broaden appeal of preserved railways, and certainly more should be done to appeal to foreign tourists, the arguement about UK demographics is arguably baseless.
To take 1980 as our abitary starting point, UK population was 56.3 million, of which approximately 52.7 million identified as white british born. In 2025, the population is projected to be 69.6 million by year end, with 73% White British, making the number 50.8 million.
If we incorrectly assume that no one other than white british people visit preserved railways, you're looking at a market decrease of 3.6% over 45 years.
Ignoring the false assumptions, growing UK tourist market and of course, the 20+% growth in overall UK population over the period, that in no way explains the rapid changes in profitability experienced since 2018.
It's impossible to know, and I wish no ill on any preserved line, but my hunch is as follows:
The generation born in roughly the twenty years following the second world war are called 'baby boomers' because the population went up so much
Stability, prosperity, employment levels so high that we actively encouraged migration (Windrush etc)
They're a demographic 'bulge' in many statistics, and a generation particularly interested in things like steam trains
In 2015, someone born in about 1950 would be around sixty five. Probably healthier than their parents were at that age, if indeed their parents did live that long (when the first 'state pensions' were induced after the first world war, most people died before age sixty five, so few qualified)
And generally wealthier - the generation that were able to take advantage of council house sales/ shares in previously nationalised industries etc, not to mention pensions
Age sixty five, possibly able to afford to take early retirement, fit enough to visit various sites a year or volunteer at one
Ten years later (i.e. today) that person is more like seventy five, time catching up. Days out may have to be rationed, funds maybe tighter after the initial boost of a lump sum from your pension scheme
Those seventy five year olds aren't being replaced as quickly by people born in the sixties, who have no memory of the lines people blame Doctor Beeching for today (whether they were on his list or not), only of the drab/utilitarian British Rail of the 1970/1980s
They don't have the same attachment to the railway, not in the same numbers anyway. And they aren't as wealthy at retirement
At the same time, lines are trying to deal with the financial realities of completing with various legislation (e.g. minimum wage, health and safety - that cafe still needs a hygiene certificate) in a world where scrapyards don't have bounteous stocks of redundant steam locomotives for spare parts
Those reaching their sixties in future may love trains because of Sodor or Hogwarts (or even Chuggington!) but they weren't from a generation where train spotting was a mass moment. The trains of
their childhood were efficient/boring , multiple units rather than tank engines
Short version - It's only a decade since 2015 but there's a big difference between sixty five and seventy five (in terms of free time/ health/ money, which are all factors in volunteering/ visiting) and the memories of trains for someone born in the 1960s will be a lot weaker than someone born in 1950
(obviously there are exceptions, as there are with all generations, I'm not trying to get into any arguments about anyone being better)