What are the chances there will be another Mac Pro in the future?
Will Apple ever make a computer that makes Siracusa happy? (and do you have the "Believe" shirt?)
IMHO - extremely little.
It is too inefficient to design a machine which _might_ have two GPU and a flock of additional drives installed into it. It just makes sense to instead design around having independent hardware in its own case, which can meet its own power/cooling needs. This has been a design goal since the trashcan Mac.
Having a PCIe bus increases bandwidth and reduces latency, but once you account for eGPU and for people who would be happy building custom solutions on platforms other than macOS, there's likely not enough identified market for a modular design.
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Never, a couple of years ago Apple gave up on the server market, that is why having Swift on Linux is so relevant for app developers.
Now they gave up on the workstation market that really enjoys their slots for all myriad of cards.
Having a thunderbolt cable salad is only for those that miss external extensions from 8 and 16 bit home computer days.
Which is clearly what Apple is nowadays focused, if you look back at the vertical integrations before the PC clones market took off.
So now if you really need a workstation, it is either Windows, or one of those systems sold with Red-Hat Enterprise/Ubuntu from IBM, Dell , HP.