Project 21631: Buyan-M missile ship

One needs to consider what equipment spaces exist beneath that location.  Ships are designed with specified areas for weapons and sensors,  while other areas support essential systems like electrical power and propulsion.  You can't completely re-arrange a ships internal equipment distribution just because you want to put a Pantsir-M on the back of the top deck. Practicality dictates that you just settle for replacing the AK-630M-2.

Pantsir has under deck equipment for missile reloads so you can't just move it around like it is just a turret... it takes up space below deck as well.

They are working on a mini SAM for their land forces which would probably also be very good for their navy too.

They are talking about an ARH missile, but an IIR guided version and also a command guided missile could be added to the range simply because command guided is much cheaper and still very effective... Pantsir and TOR are both command guided and are very accurate and effective as long as they have decent radars and even a corvette with huge AESA has good radar.