If the 8,1 was just going to be an Ultra + slots, it's puzzling why it didn't get released last year. Perhaps:
- They had enough on their plate, and the MP is a slow seller.
- They were intending to release both M2 Ultra and Extreme models, and when the latter hit problems, just went with the former. The Ultra would have been the 'entry level' model, which is why people are scratching their heads wondering what the MP really offers over the Studio.
- Apple's telemetry showed a typical 2019 MP has 192GB RAM. The M1 couldn't match that, and regardless of the difference between regular and unified RAM, forcing people to 'downgrade' would be an awkward sell.
- The M1 Ultra didn't have enough PCIe lanes to make it viable.
It will be interesting to see a PCIe block diagram for the 8,1. The Ultra + slots option had been discounted by most, on the assumption that it wouldn't have sufficient PCIe lanes. Apple made no comment about the 8,1's I/O bandwidth, so it's probably nothing to write home about; the PLX switch is likely doing much of the heavy lifting.
Now that PCIe GPUs are out of the question, though, a significant selling point of the MP over the Studio has evaporated. Many people will just go with the latter, leaving the MP as even more of a niche machine. Still, as it heavily reuses Studio components, and has a large mark up, it will likely do OK.
The 7.1 will be the new 5.1 to some extent, as it increasingly becomes a dead end and studios are willing to let go of it on the used market.
The difference will be that macOS is quickly dropping Intel (check out Sonoma's supported hardware), and the price of 7,1's is unlikely to fall fast enough to make them worth investing in. The money would be much better spent on an M2 Studio (or a PC, depending).