I actually agree with you OP, and this has been discussed already.
The thing about 7,1 is that, even for the 2019 product line-up, it was a very much a forward-thinking design that suggested a new philosophy was around the corner. A number of events coincided to reach the 7,1 design, including the promotion of John Ternus, changes in the design team and the admission (or rather, acceptance) than 6,1 was a product driven by design over real-world functionality.
As a result, Apple hadn't rearchitected a product to such great lengths in a very long time. Likewise, the development of 7,1 coincided with that of Apple Silicon, and there is next to no chance that the two departments were siloed. Therefore I don’t believe Apple would have invested so much into this industrial design with a plan to retire it some 4 years later, knowing that Silicon was right around the corner.
Yes, the profit margin this series of products will have been extraordinary, but on reflection it's clear that the Mac Pro and Pro Display XDR were targeting a difference audience than the Mac Pro of old. Therefore the higher pricing was reasonable when many large companies won't bat an eye-lid.
The release of 7,1 was also not far behind the reveal of Silicon, which should have been a marketing mishap given the performance of M1, yet the decision to go with a tower and not for instance a continuation of the iMac Pro is interesting. It's clear that Apple is committed to the tower design since it is a differentiator from the less modular Studio, Mini and 6,1 Mac Pro. PCIe cards come in a variety of functions, from audio production to video production, I/O, additional storage and much more.
Silicon will have undoubtedly impacted the sales of Mac Pro, but I honestly doubt Apple is concerned. If they do intend to continue using the 7,1 enclosure (all be it with modifications), then they have already succeeded in gaining positive feedback and reviews from the hardware. And to be clear this was never a concern for most reviewers; rather it was the Xeon chips not taking advantage of modern creative workflows and the product pricing that were the main complaints.
In short, I believe that if Apple doesn't reuse the 7,1 enclosure, then they will indeed come up with a new one fitting for Apple Silicon.