I'm pretty sure that there are two other possible main reasons for buying the Mac Pro:
One is that you have a use case that needs massive amounts of RAM, in which case clearly 128GB vs 1.5TB is a very, very nontrivial difference. (Also not sure that the M-series RAM has ECC-equivalent error correction; I assume not, which is another nontrivial need for certain use cases.)
And the other is you have a use case that actually takes advantage of $10,000+ worth of GPUs--dual Pro W6900X 32GB or dual Pro W6800X Duo 64GB. The performance on the 64-core GPU with 128GB of memory, some of which is going to be used by the CPU, is not going to be on that level even if the task is well optimized for it.
The Mac Studio is awesome, and will almost certainly be my next desktop when I eventually replace my top-of-line Intel iMac. But we've got computers to run simulations at work that need well over 128GB of RAM for decent performance, operating on 40TB datasets (stored on a RAID6 array with PCIe controller, although the storage could be handled just as well externally), and if we were to run those on a Mac rather than the current commodity hardware, we'd most definitely buy a Mac Pro, not a Mac Studio. It has nothing to do with the Intel CPU or modularity. We just need the ridiculously large amount of RAM.