I do think Apple will move to all USB-C, even on their dekstop products.
But, I hope Apple leave "classic ports" available one certain models (thus no fuss with mandatory dongles...):
- Mac mini: HDMI and USB
- Mac Pro: (two) Gb or preferably 10Gb ethernet and USB
- iMac: Gb ethernet and USB
Except for the MacBook ( which predated 'apple stable' TBv3 ) Type C has pragmatically ment TB v3 more so than "USB". ( USB is pragmatically subsumed in TB v3 controller implementation. ).
Going to pure TBv3 only a Mac Pro probably would mean a delay to the new CPU socket (and more PCIe lanes ) and chipeset ( another addtion of PCIe lanes ).
The TBv3 controllers would basically swamp the E5 v4 + 610 chipset combo when coupled to the expected SSD + other stufff workload.
E5 v5 (Skyalke-X 2066 socket ) and X299 chipset would open up the bandwidth for 3-4 TB v3 controllers ( 6-8 sockets ) along with substantially better SSD ( and possible Optane bump) and even have leftover for 10GbE (if dropped down in affordable enough range on Bill-Of-Material (BOM) price budget. )
I expect the MacBook to stop being the 'oddball' on the next upgrade and shift to TBv3 also. Type-C socket would more so mean Thunderbolt than USB in the Mac ecosystem.
That said ... that is more OCD on Apple's part than good design.
The mini couldn't do more than 4 TBv3 sockets. Going from 2 TB + 4 USB down to just 4 sockets bad. If Apple left two Type-A's that would draw tons less needless rancor.
Ditto for iMac. 4 TBv3 + 2 Type-A USB would actually be a much better match to what the vast majority of people have.
The Mac Pro... more than 6 TB sockets is a bit deranged. TB has utility but getting into extremism for extremism sake is silly. Again there are lots of Type-A devices out there in that user base and the system is absolutely
not short on space on the side of the device; even in the slightest. Nuking the TBv2 sockets (and requiring dongles) is already going to piss off folks. Even more for Type-A sockets is just going to pour gasoline on an already big bonfire.
Four TBv3 (Type-C) sockets on a Mac Pro would be useful in driving uptick in Type-C adoption. Pouring more than that probably will hurt as much as it helps over a broad segment of the population. Just drop the number of Type-A sockets... folks will get the 'hint'.