I feel like this whole topic has been diluted so much that people are picking at straws.
To me it's pretty simple - the Mac Pro will offer internal expandability (based off of the cards they touted in 2019, from Sonnet to RED to Avid) as a differentiator from the Studio. Without that single feature the Pro may as well not exist. And that's the thing; will it exist?
If Apple does retire the Pro then they need to provide a solution to or explain how users can transition from the aforementioned cards to a Studio setup. Alternatively, investing in another Pro means that Apple sees a big enough market and views cards as having an important place in Macs beyond GPUs. I would find it strange that Apple goes from touting a case filled with Avid cards, Pegasus RAIDs and internal HDD caddies to in 4 years telling customers that this is no longer the vision. History would be repeating itself again...
But one thing that's clear is that this isn't 2013. Back then many customers were reliant on 'workstation' computers to achieve things that, today, can be accomplished on a MacBook Pro. The issue with 6,1 was less to do with the thermal constraints of the hardware and more the fact Apple didn't give customers a choice of either a tower or a trashcan, with both in their product line-up. At that time, both were relevant to users.
The concept of 6,1 was sound, but it was stupidly naive for Apple to think that they could dictate the market overnight when the 5,1 Mac Pro had already languished for some time.
Today however, the Studio fulfils a lot more of those customers needs as 'professional' jobs themselves have changed significantly in that time, in addition to a far larger market of accessories and support for widely accepted standards such as USB-C.
This still doesn't make the concept of a tower archaic, but my point is that it is gradually becoming less of a requirement to achieve results many people want. Win-PCs have almost infinite PCIe support whereas the Mac does not. This is a fact whether sad or not, and Apple has to establish the value of this feature.
And given that a Studio with the Ultra chip starts from $3,999, a Pro tower will surely start at $4,999 minimum, which like the Pro Display XDR will be targeting the highest end customers, not the hobbyists.
My prediction: the Mac Pro will have limited expansion, but also an 'Extreme' chip even if contrary to the latest report.