I seem to recall that a good few years ago there was quite a debate within the UK heritage rail community about building prestige locos such as Tornado. Whether anyone likes it or not, heritage steam is now an important tourist, leisure and entertainment industry, so the question was how best to continue, given that many of the locomotives still in use have passed their century and the newest are over fifty years old. They won't last for ever, or at least if they do they will be increasingly expensive to maintain and operate, and probably increasingly unreliable. So assuming that some new builds are necessary in order to keep the industry going, what would be the best new locomotives to build?
One argument was that the main part of the industry is short branch lines usually only a few miles long with a maximum speed limit of 25 mph and very light trains of four or five coaches. They don't need fast and heavy duty locos. It was suggested that a tried and tested 0-6-0 tank engine should be selected as a standard design, modernised (possibly with the option of producing a tender version as well) and then produced in bulk (thus making it cheaper to build) in order to satisfy the foreseeable traffic demands on Britain's heritage railway lines. This plan would meet the needs of those non-purists who see heritage railways as a tourist, leisure and entertainment industry, whose punters don't know or care what is hauling their train as long as it produces smoke and makes a choo-choo noise.
On the other hand, there are the purists for whom the loco must be authentic, and must be hauling coaches of the same authenticity on a railway that looks like 1940s south western England (or whatever). They would reject the generic 0-6-0T because their interest is in preserving genuine items, just as the British Museum might reject a 20th century copy of an ancient artefact.
Then there are those involved in the main line charter industry, which can also be quite lucrative. It seems that all over the world luxury trains are making a comeback. With the privatised railway system in UK, you can run your charter train on Network Rail metals as long as you jump through all the hoops with safety cases and everything. One of the big challenges is finding paths for charter trains on the increasingly crowded network, and paying the compensation costs if your train causes delays, so speed and reliability are of the essence. Hence there is a commercial case for building new main line steam locos, as the last surviving one (9F Evening Star) is now 67 years old, and anyway that was not a high speed loco. The high speed A3 and A4 locos (one of which recently achieved 94 mph) are much older. David Wardale (surely the greatest living steam engineer) argued for building a completely new locomotive, based on some of the best of the existing designs. He produced detailed designs for his 125 mph 5AT, but couldn't make the commercial case for it. The Tornado project, to build a loco of which no originals survived, attracted private funding, as, apparently, have two separate new projects to build a P2. So the main line steam charter industry now has at least one brand new, reliable and fast locomotive as its flagship, with a couple more to follow soon.
One might also add that the main line excursions of both the old locos such as Flying Scotsman and new ones like Tornado are good publicity for the railway heritage industry as a whole - not only main line charters but the 90 or more heritage branch line operations in UK as well as the increasingly popular classic traction industry (ie preserved diesel and electric traction).
So while I understand your view, I think the heritage rail industry is more complex than you envisage, and there are commercial as well as emotional reasons for building a limited number of new steam locomotives.