The Mac Studio must be decimating Mac Pro sales, it must fit what 70-80% of Mac Pro users wanted. It's going to be incredible hard to Apple to justify development of an extended M architecture for a product used by a tiny fraction of their customers.
It wouldn't surprise me if they didn't press go on full Mac Pro development until they saw how the market reacted to the Studio.
One possibility is they reintroduce the XServe (and maybe XServe Cloud) with the larger expandable architecture to help justify its development.
The only other option is expanding the Mac Studios capability with external "stackable" peripherals, such as eGPUs.