Never is a long way off and a lot can happen between now and then.
I don't think the MM+ initiative was taken with just the current number of attractions and crowd levels in mind. When (some would say "if") more attractions are added, the system can be adjusted as appropriate. Also, significant changes in crowds could also produce changes to FP+. For example, if attendance plummets because of an economic downturn or backlash against FP+, Disney could make changes to try to lure more guests to its hotels. Significant new attractions might allow for some relaxing in the current restrictions, but could also produce larger crowds which would make even more restrictions necessary to accommodate everyone as much as possible.
The one thing I can say with almost near certainty is that there will be changes to the FP+ system over the next 1-3-5-10 years. They just might not come as quickly as some people would like. It's important to keep in mind that it is not much more than a year since the paper FP machines were removed from all parks, and there have been a lot of changes since then to accommodate offsite guests, AP holders, and cast members, as well as the introduction of 4th FPs. I can also say with near certainty that, whatever changes are made, some people will like them and some people won't.