I'm glad to see that the OP at least modified the post from the other thread to recognize that FP+ is just one portion of the whole MM+/Next Gen product.
Unless someone can correct me with more reliable information, I have been under the impression that the $1-2 billion dollars that Disney has spent on this covers a lot more than just replacing paper FP with FP+. I have thought that it also covered changes and upgrades to all of the systems that affect the customer experience (resort reservations, dining reservations, ticketing, entrance operations, DVC operations, Magical Express, Photopass, etc, etc) and allowing these systems to communicate with each other so that guests and guest relations representatives could view a guest's entire agenda in one spot, which on the customer end is in the form of My Disney Experience. A significant part of this project is the conversion from magnetic stripe media for things like tickets and room cards to the RFID chip which is by all accounts more secure and durable. But, that conversion, which includes changes to readers on thousands of resort room doors, park entrances, and point of sale locations doesn't come cheap. If it were cheap and easy, credit card issuers in the US would have already converted to RFID but, as it is, that conversion is coming very slowly. This whole thing is nowhere near as simple as Disney deciding whether to spend $1-2 billion dollars on FP+ or adding a few new attractions to the parks.
With respect to the predicted demise of the FP+ component of MM+, I don't think that will happen unless Disney decides to scrap the idea of FP altogether. I certainly don't see them going back to distributing FPs only through machines located at ride entrances that distribute paper tickets for whatever time is currently available. I suppose they could consider eliminating all advance FP reservations and going back to the first come first served system of paper FP (but doing it through magic bands instead of paper spitting machines). But, I'm betting that won't happen.
The fact that a lot of guests seem to be having trouble getting the hang of FP+ doesn't mean that paper FP was any easier for first time or infrequent guests. Paper FP was around for the better part of 15 years, and yet Disney has said (and I have no reason to doubt them) that only 50% of all guests used the system at all, meaning that the other 50% either didn't know they could use it, didn't know how to use it, or had a hard time figuring out how to fit it into their touring plans. That allowed the minority that really knew how to use the system to grab the lion's share of the FPs issued. It's that minority that now seems to have become the most vocal critics of FP+.
I firmly believe that no matter what system is in place, a certain percentage of first time or infrequent guests are going to have a hard time understanding it, especially those who don't do any research or planning in advance (no matter how much information Disney feeds them in advance of their trip). Disney's goal has to be to try to reduce that percentage as much as possible because they are never going to eliminate it.