The need for upgrading is much less today than a couple of decades ago, i.e the MBP I bought 2012 had 16 GB and 256 GB SSD, the same spec as the MBP I bought 2017. Movies, photos etc. are stored in the cloud and it's hard to find applications that really need more than 16 GB with RAM access speed, most of them are fine with swapping to an SSD. And with 3D XPoint around the corner the need for lots of RAM will be even less in the future.
The only thing worth upgrading for today is GPU/CPU.
So instead of making a modular Mac, make an external graphics card and an interface that both laptop and stationary computers can use.
I think this is what they mean when they said that the 2018 Mac Pro will be "modular."
The current Mac Pro (trashcan) is "modular" with thunderbolt. But, the bandwidth is limited for video. It's modular only in external drives. The custom GPU's are also "modular." But, of course, no third-party GPU's make one that fit it.
So, it's kinda tricky and ambiguous when they say that the new Mac Pro will be "modular."
So, logically, I think that what we can take from that "modular" statement is that the 2018 Mac Pro will be "more modular."
So, what does "more modular" mean?
I can think of TB3 and eGPU enclosure compatibility that will make it "more modular." Everything else, the trashcan Mac Pro, can be considered "modular." The RAM is "modular." The CPU is "modular." And, storage is "modular." The only thing not "modular" about the trashcan Mac Pro is the GPU.
A PC, a computer, is essentially made of "modular" parts.
So, Apple must be really trying to innovate the idea of "modular." Or, trying to make it really "modular."
But, even TB3, is limited in bandwidth and will choke modern GPU's. So, maybe, Apple is developing TB4 or a faster bandwidth way of modularizing things about PC's.
Thus, their foray into making their own GPU's. It might not be a "GPU." But, a way for GPU's to "talk" faster via modular means, like TB3, but faster.
Or, can a custom Apple GPU act as a thunderbolt-like chip to make current TB3 faster? IDK.
But, Apple is cooking something. And, their meaning of "modular," is not just making the new 2018 Mac Pro bigger, thus more modular, or putting back PCIE slots. I, mean, it's Apple, right? It can't just be putting PCIE slots back.
Am I close, Tim?