Even more than the PCI-lanes, there wasn't hardware to justify it. With Apple Silicon, the GPU is built in and you can't fill the case with cards from NVidia to make it a CUDA-monster or handle graphics beyond the (impressive) abilities of the combined CPU/GPU.
Exactly this. Apple neutered the Mac Pro by making all of its additional functionality useless.
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More than that, the Apple Silicon Mac Pro is a sad toy that was never truly worthy of the Mac Pro name by any stretch of the imagination. It doesn't even have ECC memory or upgradable RAM. IMO, Apple really should have just been honest with its pro users and said "We no longer care about you," and then they should have dropped the Mac Pro as part of the Apple Silicon transition, rather than shipping something so massively downgraded that is so many miles from being a true pro desktop machine.
Anyone who is even slightly surprised by it being discontinued was obviously not paying attention.
I disagree with Apple really should have just been honest with its pro users and said "We no longer care about you,"'.They've abandoned a very specific and shrinking segment of pro users, but the vast majority of pro users are covered by today's lineup with Mac Studio at the top. There just aren't that many things which need a traditional tower anymore. And I'd argue that almost no-one needed the Mac Pro - as you excellently explain.
One minor peeve - what is "pro" today? Most office workers can do their work just fine with the some of the cheapest equipment you get - isn't that "professional" enough? Even most developers can do most of their work on laptops these days - and if they need more horsepower, that's likely to be on the server side anyway. Don't they count? And what about project managers, lawyers, and CEOs - aren't they "pro" either?