Just like the current 13” MBP, just like the old iPhone SE, the “new” Mac Pro exists to fulfill some manufacturing contracts and/or use up existing stock of the chassis.
A “real” Apple Silicon Mac Pro will come in 2-5 years (my best guess is 3), mark my words.
The point of the pro, at least in the past, was the ability to build an incredibly powerful, expandable computer, relative to the other offerings at the time. But Apple has moved further toward integration to get wins in perf/efficiency. It seems to be a product that no longer fits in their ethos.
That you can get a machine that's just 2x as powerful as a laptop - at 2x the cost - isn't particularly compelling.
From a "real pro" machine I expect to be able to upgrade it for at least three to five years. These things cost thousands of dollars ffs.
High speed interconnect, maybe proprietary.
Potentially multiple sockets if they can’t scale their current design.
Fast, redundant, hot swappable storage.
They have the source and can compile for themselves. I doubt x86 support will truly leave while it’s still the only way to do servers.
There's no way Apple would invest in all that technology to sell its least popular Mac.
It is very difficult to predict which parts are going to be successful.
And if they manage to create a product definitively better that competitors (like the current gen ARM macbooks), they are going to sell a lot more than 50,000 per year. There are single companies which can afford to buy 50,000 units.
They made the 14 and 16, but they presumably still had plenty of 13" frames left, so they're just selling both. Jobs-era Apple likely wouldn't have done that, as it causes the product lineup to be a good bit less clear, but in Cook-era Apple, that's an acceptable tradeoff.