Hi,

9 speed will always be under pressure as the first rear size than cannot
use a generic 5 to 8 speed chain, but it will always work with a 10 speed
crank and 10 speed chain.

So long term expect 10 speed chains and cranks to predominate over
9 speed in terms of the choices available, it is common sense that
it will pan out that way, which will lead to more choice of 10 speed
rears and 10 speed shifters in performance parts.

It makes sense that bikes group around 8, 10 or 12 speeds and
that 9 and 11 speed are somewhat superfluous to requirements.

6 and 7 speed will last as long as freewheels, a long time.
5 speed is now effectively dead AFAICT.

8 speed needs a freehub, but uses the standard chain and cranks.
(7.1/7.2mm width). Its future is assured.

6.8mm (9 speed) chains versus 6.2mm (10 speed) chains long term not so IMO.

I agree with the OP, I think 9 speed will endure a slow lingering death.

rgds, sreten.