Chances are there will be a second 4 port, more powerful MBP launched along with the 16”, so I’m going to say that the larger discs will be available.
I’m not sure if its technical limitations or a deliberate decision based on historic sales of the MBP (and Air). Today was about getting the low and mid-end laptops transitioned (biggest sellers maybe?) and the high-end stuff will follow. Specs are probably based on what’s sold best in the last few years?
If its technical, I’m sure one of the Apple podcasts will get an answer (officially or unofficially) soon.