Well, I doubt the aluminum frame would like the 357 magnum which would eliminate the 650. But they could make one in titanium. I guess they figure that the 651 Ti is pretty much the same thing.
As far as the 850UL, your guess is as good as mine. I wish that they did. But once again, they probaby figure that the 851UL is close.
As I'm thinking about this it could partially be based on Smith's sales of similar guns. They didn't make their Centennial (equal to the 650/850) for many years but the Bodyguard (equal to the 651/851) was still in production.