...well, if you go back and read the transcript of the 2017 "apology" they were mainly apologising for the dead-end "thermal" design which prevented them from offering updated models with new CPU and GPU, were using "modular" to mean "anything without a built-in display" and I don't think they promised anything about user upgradeability - just their ability to produce updated models. Of course, history shows that we did get a fully modular/upgradeable PCIe based system but I got the impression that they hadn't decided at the time and were choosing their words carefully.Sadly I agree, but it doesn’t change my statement. Apple made an apology about the trashcan, then touted “modular design” and “upgradability”.
My guess is that the whole thing was triggered by pushback against the iMac Pro (which would have already been in development then) being the "new Mac pro".
When it comes down to internal politics, kicking the can a few years down the road is often the result.Touting upgradability in a platform they knew they were going to trash five years later
Actually, it makes more sense to produce an upgradeable machine in those circumstances. The design is probably good for a few more years - they even updated the GPU options last year. They could even, possibly, use the MPX bus as a way to add Apple Silicon "compute modules".
I suspect that, if Apple announced that they were going to support the Intel Mac Pro for a few more years rather than discontinue it in favour of Apple Silicon, a lot of existing Mac Pro users would be quite happy.
Problem is that they haven't said anything (maybe a few big customers know something under NDA but we'll never know) so as far as anybody knows, the 2019 MP is going to be discontinued at short notice when the new shiny comes out.