While I could see the Mini taking over the role of the studio, I'm assuming the Studio came out for a reason, which was to hit that market that doesn't want an iMac and doesn't want a Mac Pro.
Future 3nm processors might make it thermally feasible to fit Pro/Max processors in a "Mini" enclosure, with the Studio Ultra (... and "Extreme" if that ever happens) becoming the "Mac Pro". The "Studio" would be gone, but not really failed.
If anything seems likely to be 'one and done' I think it might be the forthcoming Mac Pro.
I wouldn't bet on that not being a "none and done"... The "more about the Mac Pro later" could be a presentation on why you should stop worrying and love your Studio Ultra. The Studio is
already the spiritual (and far more sensible) successor to the Trashcan - which is clearly the direction that some people at Apple wanted to take the Mac Pro.
The 2019 Pro is by far the most problematic Mac to transition. As far as stitching Max dies together, the 2x Ultra already seems to be into diminishing returns on CPU and GPU - beyond that you're in to a horribly expensive way of expanding RAM and I/O bandwidth. Creating a "Xeon-W killer" with ARM cores, massive (external) RAM capacity and lots of PCIe is feasible but would be a huge investment
just for the Mac Pro market and would dump many of the defining, successful features of the M1/M2 such as on-package unified RAM, powerful, integrated, Metal-optimised GPUs. ARM would being better power/performance - to the "powerful stand-alone PCIe workstation" section of the market where power consumption and size just aren't big concern.
The 2019 Intel Mac Pro is still as viable as it ever was if your workflow needs shedloads of RAM and big. meaty PCIe GPUs, and would be fairly easy to refresh with Intel and/or AMD's latest chippery. For those able to "think different" a cluster of Studio Max/Ultras (maybe Apple could do a rackmount version of the Studio and call it "Mac Pro") linked up with Thunderbolt seems like a more appropriate use of the Apple Silicon we've seen so far.