I’ll say what I think on this matter. Everything below is a summary of my thoughts and it is msot certainly not exactly what others will feel.
So I’m 15. I am into railway photography, mainline and heritage. I was brought up with a large interest in steam, and I’ve more recently started to grow my interest in diesel locos. I have a group of friends who live locally and share my interest and we travel and attend various rail events together.
So is there a decline in enthusiasts? No, however they are different to what some see as the stereotypical trainspotter or enthusiasts. There is a much higher focus on photography and videography, and much less standing with a notepad. However, many young enthusiasts including myself still like to note our traction and sighted locos. A lot of content is posted on the likes of Facebook, Instagram, YouTube and Flickr, some of which have large communities building and connecting all different young enthusiasts. There is a large presence of enthusiasts of my age at diesel galas, and less so at steam galas usually (younger kids with parents I find). With less loco hauled and slam door stock, mainline interest is dropping as units are, for many, less interesting.
Will the number of enthusiasts decline in the future? I think so, yes. Pathing issues and trespassing are pushing charters off the mainline. Coal issues seem likely to stop or at least heavily limit steam altogether in the not too distant future. Diesel interest will eventually go away I reckon, mainly given the fact that a time will come where diesel locos won’t be coming off the mainline and into preservation (because of electrification etc), and I hope that there won’t be Azuma preservation in 30 years time.