High speed networking cards.
High speed video encoders/decoders, maybe?
Professional digital video capture and output.
Professional audio I/O boards.
Massive and super fast storage cards.
... lots of exceptionally niche products with very few users.
Speaking as someone who works in film and television and has used cards like the ones you mentioned over many years…
You typically don’t need video capture cards AND audio I/O boards in the same machine, since ingest/edit is separated from audio mixing. On a television show/movie, your editor is not ALSO your sound mixer. And if you’re working on a solo passionate project, you can do plenty of mixing or editing without those cards. Anyway, point is… You don’t need a capture card AND an audio I/O card for something like ProTools in a professional environment.
For super fast storage, you’ll always do better with a large external RAID (particularly in a shared user environment). And/or working with proxy media. Having super fast storage cards (which still won’t hold all your full-res media because they’re lower capacity than a RAID) isn’t that useful in a professional setting.
The Apple Silicon chips have built-in hardware decoding for video codecs. Remember how the Intel Mac Pro had an Accelerator card? Those are no longer necessary with Apple Silicon. There’s a reason why Apple doesn’t offer the Accelerator card for its Apple Silicon Mac Pro anymore.
As for high-speed network cards… or video cards… or audio I/O cards… These can all be put into external Thunderbolt enclosures attached to a Mac Studio.
And the Mac Studio is currently just as powerful as the Mac Pro. And cheaper.
And that’s the fundamental problem.
There’s nothing special about the Mac Pro. Sure, maybe having the convenience of PCIe cards in the machine rather than using a Thunderbolt PCIe enclosure is nice… But you’re not gonna need that many cards in a professional environment. Not anymore.
Maaaaybe if Apple allowed for compatibility with high-end GPU’s that might change things. But I don’t see that happening.
Fundamentally, Apple didn’t know what to do with the Mac Pro. So they stuck a Mac Pro in an oversized enclosure from 2019 and called it a day.
They should do better. Offer more power. And allow for GPU support for extra speed… Which would occupy at least one PCIe slot when I might ALSO need a high speed network card and a capture card. Then, suddenly, I need multiple PCIe slots.
But right now, a Mac Studio with an external Thunderbolt PCIe enclosure is just as capable and far cheaper.