I had a U-Audio PCI card in a Mac Pro during the Intel era of Macs. It was a chip to run their software plugins and the plugins are top of the line. I have a U-Audio box that runs over Thunderbolt now. I know there are people who need device slots, but it's vanishingly few. I'm disappointed that this category of machine is going away, but it stopped being for me in the Apple Silicon era.

so many peripherals now come in external boxes that communicate _incredibly quickly_ over Thunderbolt 4/5 that the need for PCIe is marginal, while the cost to support it is significant.