Do you have a source that says 100% of people use it? I've never heard numbers that high.
Even so, the fairest system is the one that creates equal opportunity, not equal results IMO. Equal opportunity is everyone having the chance to get to a park at opening and collect FP, choosing not to does not mean it's unfair. Equal opportunity is not (again, my opinion), different booking windows depending on your resort, different opportunities depending on your ticket, technology use outside the parks being required for best results, and previous park knowledge.
Am I complaining that FP+ is unfair? No, not really, but when you discuss the fairest system possible I don't think placing more barriers on guests that some overcome easier than others (to the point that it's impossible for some people to overcome) is the answer.
Again, the most equitable way of doing things is one ride per headliner per person per visit, and no FP allowed (because not everyone can get one). Clearly that is not what this is about.