I had a model 70 briefly when they were new. And now, with young grandkids growing up too fast, I wish I still had it!
I do recall the guns seemed quite pricey for what they were, though - especially the complex double-piston model 72 - and the lack of a safety may have presented regulatory or liability limitations. And to be honest, Diana's marketing in those days was awful - RWS's catalogs and magazine ads for the US, at least, were laughably amateurish compared to Beeman's stuff. So, possibly the models 70 and 72 were not a reflection of the true market potential.
So to answer your original question - IMHO, yes, there would be a market for a good small springer at reasonable cost, and I think a similarly converted barrel-cociking pistol action would be a good way to go about it.