Good question, and I don't know

Lots of research shows people spend more money when using cash alternatives, so even moving people over to MBs would increase their spending, if only slightly.
We have seen a mixed bag on what FP+ has done to the parks. Some people have reported being in the parks for shorter periods of time, Disney said they were able to accommodate several thousand more people on Christmas per day than before FP+ (I guess from cycling people through the park faster) ... its really difficult to know how that pans out. Sure that sounds great for Christmas, assuming people still spend cash while they are in the park, more people in and out, potentially more cash spent. But the rest of the year, if you aren't getting any -more- people through the park, but they -are- spending less time in the park, they might be spending less money in the parks as well. This is not a good thing.
Disney has said their per person spending has risen over the last couple years, but it has risen on rising prices, including tickets, resort rooms, merch and food and bev, all those prices have increased, which makes it very difficult to isolate the impact of MM+ and FP+ on revenue, which I honestly think might have been the point of the presentations we have seen to investors. No one wants to say ... we spent over a billion on this thing and aren't really getting any returns from it ...
I do wonder what they have done cast side as a result of FP+, again we have seen mixed reports of under staffing, terrible wash rooms, etc. We of course have no real way to know if there is any correlation there, are they trying to cut staffing based on the number of people reserving FPs in a give park, on a given day, even over given times of day as a predictor of crowd levels ? Maybe they are, maybe that's saving them some money.
I am hopeful for what MM+ offers for the future and next gen rides, although I read somewhere that basically not much of that is going to materialize, at least not in the next half decade or so, that imagineers were all gung ho about the possibilities but that basically their wildest (and most awesome) ideas have been quashed by management. What we are really going to see is very light and superficial uses of MM+ on attractions, but who knows, the potential is there, just a matter of it being taken advantage of.