If a ride were to be treated as FP only for the whole day, that would mean that when the rope drops nobody would be allowed to walk right to the attraction and ride unless they had a FP. Just think for a second about how ridiculous this is. Based on our own experience and recent reports from Josh at easywdw and other posters on this board, if you walk directly to a ride like Thunder Mountan at rope drop you can ride multiple times with no wait and without a FP. If Disney were to adopt a FP only policy, that would mean that for a certain period of time, they would be operating rides at significantly less than 100% of capacity even though there are plenty of people in the park who would love to fill that capacity. The same thing would happen in the final hour or hours before closing when the capacity exceeds the demand. Depending on how crowded the park is, that could continue for several hours and, for many attractions, it would continue all day. No matter how sinister you think Disney is, you can't possibly think that is what they want to do.
A lot of posters on this board have said that they are unwilling to wait in a standby line of more than 20-30 minutes most of the time. If that is your attitude, a number of the most popular attractions in the parks are already de facto reservation only, and have been that way for years. Using TSMM as the most obvious example, the standby lines there are an hour or longer except at the very beginning and very end of the day. If Disney wants to eliminate my right to wait in a standby line that long, I really don't care because I have forfeited that right voluntarily. If Disney made Anna and Elsa FP only, would that really be a significant development for very many guests?
The absolutely worst case scenario I could see is that Disney would allocate 100% of ride capacity to FPs and the standby line would become a true standby that would be used only to fill gaps that might arise in the arrival of guests with FPs. There would be no guarantee that someone in the standby line would ever get to ride. I really don't see Disney doing that either, but it wouldn't really bother me if they did. I just don't think there are that many rides that would "sell out" of FPs, especially in the first and last few hours of the day, unless Disney significantly increased the number of FPs every guest can make in advance and/ or hold at one time.
The thing that makes this all possible, from a public relations POV, IMO, is the addition of the 4th FP. It moves the standby lines to the app. I don't think they will ever make all rides FP+ ONLY or even FP+ for that matter. But for some, I could see it happening now that they have done this test with these character meets. Maybe TSMM, Soarin', TT, 7DMT, A&E. In other words, they can do it as they see fit. If they did, the 4th FP gives them an out, they never have to say to anyone, sorry no standby, they can say, sure, go to the kiosk or hopefully the app. It creates a virtual standby line while getting rid of the physical standby line. I don't think they want people standing in hour-long+ lines ever. They have to manage those lines, and more importantly, those people could be spending money.
They are already giving priority to FP+ in ways unlike the old FP, which worked on ratios.
As far as rope drop, again, I don't think they have to put it on all of the rides. But some attractions and rides build a long standby line immediately...those are the ones who would be prime targets for a FP+ ONLY designation. As far as managing close of the park, again, let people use their 4th rolling FP. And of course, you can also give some leeway to the CMs to wave people through at very low crowd times.
I also don't think they want people standing in true standby lines, like what we see for Anna and Elsa.
I also think that the 7DMT reservations are through the roof, and that is part of this possible change. That area is already tight. They don't want a repeat of the Anna and Elsa situation up there.
Just my theory, I hope I'm wrong.
Under Disney's new system the only walk ins will be gift shops. 
