Well, this is just my opinion, I've never experienced "Legacy" FP at WDW, and haven't been to DL yet (but soon!), but I think it's exactly in the efficiency of MP that it will reduce the quantity for the paper FP. I'm going to use bogus numbers to make it easier to picture, and let's imagine each individual pullsjust one FP.
Let's say, right now, 100 people walk into the park at rope drop and head to the FP machine for an attraction. Person 1 gets to the machine, puts in their ticket, gets a paper FP for timeslot 1, and walks away. Person 2 then walks up and gets timeslot 2, etc. This takes a certain amount of time before person 100 gets to the front to put their ticket into the machine and gets a FP for timeslot 100.
With MP, if those same 100 people walk to the same attraction at rope drop, you could also have 50 other people who have paid for MP and are using the app to get FP at the same time. By the time person 1 reaches the machine to pull out the paper fast pass, maybe 10 people of the MP group have snagged one on their phone? So the walking person gets timeslot 11. When person 2 enters their ticket into the machine, maybe another 10 of the FP group pulled theirs, so walking person 2 is now up to timeslot 22.
While the same total number of FP will get distributed throughout the day, since multiple people can pull FP on their phones at the same time, FPs will run out faster. Unless Disney has somehow hired a tech genius and only a certain amount of FPs will be available through the app (from my example above, say only timeslots 5-10-15-20... are available through the app, and all other time slots are in the paper machines). But I don't think that's likely. Especially that they want people to use the app for FP, as that means more revenue for them.
All that being said, I'm guilty, if it comes out before our trip (which I doubt), we'll most likely try it. There are just 2 of us, only 2 days, so $40 isn't that much for the convenience (plus PhotoPass)