I am still surprised Apple never introduced a Mac Studio with a Mac Studio Extension chassis option for limited internal PCI cards (no GPUs) and lots of HDD storage.
Why bother when 3rd parties are going to do it? Apple doing a Thunderbolt enclosure would only inhibit drawing more players to fill that role.
The extension box is primarily just a 'container'. Is the grand Apple Industrial design, going to be happy doing just a "container" for cards that require wires ( given Apple's 'war on wires' outlook ). Yes, Apple does covers for their phones , but also largely say don't need one. And those are at a complete diferent unit volume scale. Even still the number of 3rd party options greatly outnumber Apple's, so there is little suppression of compeition.
They probably thought that PCI slots are no longer needed and that physical HDDs will all be replaced by SSDs soon - and then AI killed their plans and SSDs never really came down in price nor increased in capacity much.
No. SSDs did come down in $/TB. Apple never priced their own that way. They kept the $/TB the same and used storage to pad margins. HDDs have disappeared from most consumer PC devices.
The 'problem' more so is that Apple thinks most folks are happy enough with the one , and only one, drive that Apple provides. And that it is 'fast enough' for vast majority of folks. And maybe one more x4 PCI-e v3 SSD is enough for most people.
Apple knows some folks need PCI-e slots. Otherwise they wouldn't have released the MP 2023. The question is how many folks and how many units they would buy. Thunderbolt delivers PCI-e to any Mac with a Thunderbolt port. So a wide variety of PCI-e cards can be used by those that need them. Thunderbolt v5 only expands that coverage to a wider set.
Apple also trimmed off one whole class of high bandwidth , low latency PCI-e cards by merging the GPU with the SoC. The GPU gets much higher bandwidth and lower latency with that trade-off. Thunderbolt is also weaved in ( No MPX connector 'work around' data problem. )
There is a class of A/V cards left but the MP 2023 covers most of them and that future TBv5 Studio can cover a significant number of those. Apple could also maybe add a Occu-Link x8 connection later which would cover even more.