It not so easy just to add a diamond cantilever to an existing cartridge without tuning the cartridge, how does one determine that a diamond cantilever makes a difference, a cartridge has to be voiced to work together. I have have added diamond cantilever's in the past that have been disaster. I have had a diamond cantilever on a Koetsu Urushi it was a waste of time. My conclusion is that a cartridge has to be voiced to the materials used, I am sure if top wing come up with a new model with a diamond cantilever they may be able to come up with something tasty but I am sure they will look at other aspects of voicing the cartridge at the same time. I once asked a manufacturer to include a shappire cantilever to a design I had to send the cartridge back so many times to play around it it the balance was changed beyond any sence.
I agree that blindly swapping to a diamond/ruby/sapphire cantilever may not make sense. But designing around a diamond or sapphire cantilever does because of the technical merits. They are normally one piece, i.e. cantilever plus tip. So no glue joint and less mass. This should then translate to lower jitter, and therefore better tracking capability and lower overall distortion.
I believe there are only a few cartridges which have been designed with one piece diamond cantilever/stylus, which would have to be made with the stylus profile machined in the end of the cantilever as a single piece. It is more common for cartridges which use a diamond cantilever (like the recent top designs from Ortofon) to still have a separate diamond stylus which is attached to the end of a diamond cantilever, not a single piece.