Here is the direct quote from Phil Schiller (bold is mine), from Apple's Mac Pro mea culpa press get together:
We’re not going to get into exactly what stage we’re in, just that we told the team to take the time to do something really great. To do something that can be supported for a long time with customers with updates and upgrades throughout the years. We’ll take the time it takes to do that. The current Mac Pro, as we’ve said a few times, was constrained thermally and it restricted our ability to upgrade it. And for that, we’re sorry to disappoint customers who wanted that, and we’ve asked the team to go and re-architect and design something great for the future that those Mac Pro customers who want more expandability, more upgradability in the future. It’ll meet more of those needs.
Apple rarely apologizes, much less uses an entire press event to apologize. So my interpretation is that they wouldn't have done this unless they were serious about that mistake. Phil expressly mentioned expandability and upgradability as issues, and that they would fix them in the next iteration.
- So when people say that there won't be expandability or upgradability, I think they are incorrect because this is contrary to Phil's direct quote. I suppose it's possible, but it would be straight up lying on Phil's part.
- When people say that there will be expandability or upgradability, but it will just be via Thunderbolt, I think they are incorrect as well. The 2013 already had that, and the whole point of this press event was to indicate change from that mistake.
Now that being said, it doesn't guarantee that expandability or upgradability will come via industry standards like PCIe slots, MXM sockets, SATA connectors, DDR slots, M.2 slots, 2.5/3.5/5.25 drive bays, or whatever. They mention "modular" Mac pro, which is
very open to interpretation. It might be an ongoing stream of updated Apple custom modules and boards, similar to the nMP's GPUs--that way Apple could support expansions and upgrades, while still retaining complete hardware control and capturing ongoing revenue.