Few things.. You cling to the idea that if Apple do this, the market will follow and offer everything one could possible want, everything will be perfect, suddenly everyone will run around with USB'C thumbdrives etc..
I'm not saying that won't happen, but we are FAR from that. We are taking 5 years, at-least, in the meantime you will be frustrated with your mountain of adapters.
And if you want this shift to USB'C to happen, the market leaders would have to do the same thing, Dell, HP, Lenovo etc. Apple does not have a big enough piece of the market to really push new standards. Look at firewire, Thunderbold, Lightning etc..
I agree that USB'C is versatile and useful. But it's too soon to put all your chips in one bag. One day it will probably be the de-facto standard for all computing needs, but not today, not tomorrow.. This will be a computer before its time i'm afraid.
And just for the record, USB'3 is not legacy.. Just because its been on the market a few years does not make it old and obsolete.
You also mention an adapter that will do many things, sure, those exist and are useful. But the question remains, I should not have to buy an adapter to use a run-of-the-mill USB-drive in a professional grade machine.
In the back of my mind, I wonder if you somehow work for Apple, so convinced with you and your company's superiority in the market, that there is no question in your mind that everyone will follow your lead.
I'm not so sure.