If it didn't sell, the pumps would have disappeared long ago. Remember the Porterhouse Weiss Bier?
I am a fan of TSB, would love to see seasonal variation on the cask and would be very surprised if other TSB drinkers would not. If you are willing to drink one cask beer you will try another. Having said that, I think TSB, at 3.7% ABV, fills a “sensible pint” niche, which might explain why the porterhouse could be reluctant to mess with it.
Séan