I haven't been to WDW in a long time, but last time I was there I used FP in the time with no problems, had no ADRs(and never will) and was with 7 people total(2 kids) over July 4th, but we never took breaks. This was before TSM, Soarin, Mission Space and EE, we saw all we wanted in 1 day without having to rush all that much. I know things have changed a lot since then but here's my opinion on the whole late FP use.
I was not planning on using my FPs late for my honeymoon trip in June unless I got held up in a stand by line, show or park traffic that forced me to arrive say 20 mins late. It was more of a backup plan to know FPs could be used late. We have no kids, no ADRs and plan on touring commando style starting at RD, getting FP immediately for headliner and riding non FP rides to wait for window while getting second FP before we ride our FP ride, we only really plan on riding each ride once, unless we see a short wait later on for something we really liked,I don't see us having any issues for arriving on time.
Basically from reading this thread all the intentional late FP users want to have their cake and eat it too, meaning want to keep using their FPs late, keep all their ADRs, keep taking breaks, and avoid long SB lines. They don't want to change their touring style and ok it might lessen some of your magic but some of us are just glad we can even go to WDW once every 5-10 yrs, and even get FPs for some attractions as I have read they run out for some very early. You will need to make adjustments and who knows what kind of plan Disney has in store for their new system of lessening wait time.
As far as the zero sum theory while mathematically it works, some guests will benefit earlier but the rule abiding FP users will wait longer later as well as SB guests, so it's not zero sum for all guests experiences.
Just my 2 cents, bash or critique if you wish.