They made three assumptions, none of which panned out, but took years to become clear:
There was a more fundamental problem: they completely and unnecessarily bet the entire "Pro" farm on the assumptions you mentioned. They let the preceding Cheesegrater stagnate without significant updates for years and completely discontinued it the moment the new Pro came out (on this side of the pond, the grater was dropped before the nMP was even announced, for want of a cheap plastic fan guard to meet an EU reg that had been signalled well in advance). Users who had been waiting to update their cheesegraters were expected to instantly drop what they were doing and embrace a radical new paradigm, whether or not it actually fit their needs.
They've just done exactly the same thing with the MacBook Pro - the new MBP is clearly what
some people want, and may be brilliant as USB-C/TB3 gains traction, but everybody else is stiffed with the option of a 2-year-old base model with most of the options discontinued and no reduction in price (in many countries, the price went up).
Contrast this with: (1) the launch of the 2012 Retina MBP - with the controversial loss of optical drive, ethernet and firewire.
At the same time, they updated the classic MBP to the same CPU as the Retina + USB 3 so if the Retina wasn't for you, just yet, you had a viable alternative. (2) the MacBook Air, which ran alongside the rest of the range until it had been around for a year or so and been through a major revamp. Even the famous demise of the floppy happened over a longer time scale than people seem to remember today, and had been optional extras on PowerBooks for a while before the iMac.
If your magical new concept really is that magical, have the confidence people will buy it in preference to the old concept when they're ready to change their workflow. Don't force your vision on people by suddenly taking away the alternatives, and certainly don't let your old models become a laughing stock while you work on your new shiny thing.