See, the more I think about it, the more I think that's why they might want to keep them both running. They don't want to go all in on PP and risk their lines getting out of control on waits at random times and then having even more complaints and bad press, so they have this as a small bit to increase profits while they can still mainly control those lines. And while I'm not sure I buy the FOMO, I do buy the idea that people may buy MP to feel like they are getting a true upgrade (while they can't justify the PP expense). Now, eventually you could be right. But I think the more plausible idea could be PP become MP with a $150 price tag or so, and PP becomes more like unlimited (with a higher price tag), similar to what Universal does (since WDW has been kind of a follower recently).
BUT, I think they are a bit from that. If I was testing the waters for that, the first thing I would be doing is collecting the data on when people are using the PP for each ride, and seeing how many per hour you can expect. Then increase the number of passes available and recalibrate. Do it again and again til you have enough data to really estimate how your lines could be impacted if you really open it up, and how often you need to send from each line (just cause they have PP doesn't mean you need to send 75-25 from that line). THEN you consider changing the system up based on your modeling. But once you change it, you better be ready. Cause like I said, the price people are paying for vacations, if you miss the mark on your wait times (or people don't wait in line cause of a wildly inaccurate time), you will see your guest satisfaction plummet even more.