And it will continue to be "a future thing" until USB-C becomes as ubiquitous as USB-A, which I doubt will happen for at least another 5 years, and is why Apple should have made rMBP machines during this change-over time "BRIDGE" machines, bridging today's USB-A with tomorrow's USB-C. And as I've repeatedly pointed out, there's plenty of room on the 15" rMBP to accomplish that simple and inexpensive feat.
USB-A really cannot be compared with Firewire or other proprietary connectivity which wasn't really used by the entire planet like USB-A presently is.
Again, this is the MacBook PRO, not the AIR, not the MacBook. People who own a late 2016 or later rMBP 15" and who are happy with it would probably be just as happy if Apple had put a Retina display in the MacBook AIR and sold them that instead. And that, my friends, is where Apple screwed up royally. They should have gutted the AIR and MacBook lines while leaving more features (important "bridge" features) in the upper end "Pro" machine.