Maybe there are some who would do it, but I think maybe cancelling a trip because your Illuminations fastpass was cancelled is an overreaction...of course they still can if they want to, provided they eat the cost. They could even try calling and say that they expect a refund on the entire trip because their Illuminations fastpass was cancelled, but Disney doesn't have to give it to them. And no, it's not anything like ordering a car and then when you pick it up it's a different car. And yes, it's printed everywhere that anything regarding WDW is subject to change without notice (legal CYA, so you can't do anything about it legallly like suing them). Most folks just don't read that part when they are looking at the website or printed info.
It's that guests don't like that something you were looking forward to was changed. Nobody likes that. But Disney does reserve the right to do it.
And not surprisingly you missed the entire point of the post and only cut out a tiny, out of context, slice to respond to.
A) I didn't say a different car, I said a different colour, same car, so why do you care ... right ? I mean, its only thousands of dollars spent and one tiny detail different. Wont change your enjoyment of the care, will it ?
B) Does Disney advertise that they can pull the rug out from under you ? Is that actually part of the commercials they air ? Is it in big printing when you go to book your trip. Is it in large print in their brochures and ads about how awesome the system is that ... "It's Magical that you can now book your Top 3 rides and be sure you will enjoy them, unless we decide to cancel one or some or all of them at any point as we reserve the right to do !!!!

No, its not. Its in the tiny print, its not part of the sales pitch, and Disney would rather people didn't know they can and will do this.
C) And that's exactly the point, Disney is trying to make you LOCK in to your trip and really, for most people beyond the 60 days they are locked in, if you have flights booked and time off work booked etc. To back out is going to cost you. So Disney says ... you go ahead and lock in to your trip, and at 60 days we are going to assure you that your trip is awesome by allowing you to book your FPs so you "Know"TM you will have your three favourite ride selections. And then after that, when there is no looking back, we are going to change what you booked and paid for and too bad if you don't like it because "Know"TM Actually means that we reserve the right to change any of the things you thought you were getting with your trip and offer you nothing (sure some people get other FPs, but some people just lose their FPs) and we can keep your money anyway.
Tell me ... at what point does this become NOT ok with you? If Disney wanted to, you could show up for your vacation and they could say "Sorry all the parks are at capacity today" (And that capacity could be 10 people) or "We have decided to back fill all the pools on the resorts" or "Sorry but we decided to burn the castle down" because none of that is guaranteed right ? People book trips, and FPs, and they are told in big bright letters and loud voices that THIS is what they will get for their thousands of dollars. And yes changes to that, especially when getting close to your trip dates, are bogus. This is even more true considering Disney's continued effort to make us, as far as they can, "lock in" to our trips.