That sounds like a wonderful eutopia, but I don't believe it. History tells me these promises will not be kept, and the compatibility will not work like they say.
Since smartphones and media players became ubiquitous, I have seen smartphones and media players (and the like) with the following ports:
;
;
;
;
; and some proprietary USB ports like HTC's ExtMicro.
Between Blackberry, HTC, Samsung, LG, Palm, Motorola, etc, over the past ~10 years, there have been too many different ports. Sure, the Micro-USB seems to have become to the dominant one, but only in the past 3 or so years, but even Samsung over the past 3 years seems to have used 2 different ports.
So now USB comes out with USB-C and tells us that this is the one, this everyone should use all the time. Ok, I've heard that before.
Why should I believe them this time, when the last half-dozen times it didn't turn out to be true?
And, not even a year goes by, and they start adding more alternate modes on USB-C. As I was considering to buy the retina Macbook for a bit, I had looked up what the big deal was with alternate modes, and it turns out it's a huge mess. Some USB-C hubs are compatible with Apple Macbooks, some aren't; some hubs are compatible with HPs laptops, some aren't; there is no hub that has USB-A ports and an HDMI port that is compatible with all USB-C based laptops, because not everyone implemented the same alternate mode specifications.
You say that USB tries to be backwards compatible, but they do very little to be forward compatible. In theory, the 2015 Macbook's USB-C should work with any USB-C device that is released in 2018, but I highly doubt that will be the case. It doesn't even work with all USB-C devices that were released in 2016.
I agree with you that proprietary tends to be less desirable. I would prefer it if USB and Intel and all those heads of companies on the USB-IF's board got it together, but they don't have it together.
Meanwhile, despite being proprietary, Apple has used only 2 ports (30-pin dock connector, and Lightning) in the same time period. With a few minor exceptions related to FireWire in the 30-pin port, those two ports were fully backwards and forwards compatible. A speaker dock purchased in 2006 would work with an iphone from 2011, and likewise an iphone purchased in 2008 would work with a speaker dock from 2011. Likewise with Lightning, all Lightning accessories work with pretty much all Lightning devices no matter when they were made.
Since 2003, 2 ports, and pretty much full compatibility. That is what Apple's proprietary systems have done. USB might have a lot of promise, but historically it has never come close to matching that sort of good user experience. Apple can do this because they control the hardware and the software. USB can't do this because there isn't much they can do to punish accessory makers for not following the specs perfectly.
I have seen USB change way too often to take them at their word that USB-C is the solution to all my problems. I'll stick to the proprietary standard that has historically not caused me any problems for now.