Under FP+, you can guarantee yourself rides on headliners months in advance. But with tiering and ride reallocation, you cannot ride headliners as often as you used to, assuming that what you are used to is constant headliners. If you are in the "I only rode Test Track once per day anyway, so FP+ hasn't hurt me" crowd, then none of this has any meaning.
But if you were in the "my trips to Epcot were not complete unless I rode Test Track 4 times a day" crowd, then there is no question that FP+ has changed the game. So your first statement..."under FP+ you can no longer enjoy the type of WDW vacation that you got used to under the old FastPass system" becomes true for that person. So they adapt and find a new way to enjoy WDW, using a system that works quite well at doing what it is supposed to do, making your second statement..."the new system works very well" equally true. Why do so many people have a hard time understanding that FP+
really has changed the way headliner commandos enjoy the parks?
That is an intended consequence. Disney intentionally built a system that discourages, if not obliterates, the ability to ride headliners over and over again, leaving other guests with no opportunity to experience those attractions without huge waits. And when someone points out that the intended consequence is actually coming to fruition, some FP+ supporter has to chime in and say: "No it hasn't". But if you love the new system, you should love it for all of its consequences, and one of those consequences is ride redistribution.
Take from the commandos. Give to the leisure crowd. It was intended to do this, and it is doing it. The OP is now positing a way to enjoy WDW under new and different circumstances. Would I want to ride 3 or 4 rides a day and call it quits? No. But I have no problem with anyone who wants to do that, especially if they have the luxury of multiple weeks to enjoy WDW. Heck. I have a brother who used to work there and he did that all the time. Go to park. Ride a few rides. Leave. In some circles, that was an envious life. The OP says that he is going to try it and he is called everything from illogical to idiotic.