I am sure there are many more people out there that don't even use forums so numbers would be a hard thing to guess.
I know just off the top of my head I have 20 pieces of rolling stock that I could convert with a cranked NEM. I would probably have more but have decided to no longer purchase items that have cranked tension locks. I would think I have a relatively small collection of rolling stock compared to others.
I have no idea what tooling costs would be but who is to say they even know about cranked tension locks? How did they know there was a market for NEM Kadees.
As with anything if they don't here from us they won't make it, be it Kadees, locos, rolling stoc or whatever it happens to be.