It's not dying. Not even slightly.
I was gone for a good 8 years, came back to find the likes of force racing have branched out, KAD have a wider portfolio, arc angels the same. Minispares and minisport do the same old things. Then there's all the engine conversions out there, it isn't just watsons anymore. There's half a dozen choices.
Are they less accessible to young drivers, certainly. The price has gone up for a runabout. That also means they're more likely to be looked after with a reasonable budget and they're less likely to be driven daily. A great thing if you ask me.... although I wouldn't have agreed if I was the young'n wishing to daily one like I was 10 years+ ago