One assumption that many are making is that everyone will use their 3 on headliners. That's just not the case. While TSMM and Soarin being the exception, not all of the headliners are family friendly, so not all will book those. I'll bet the majority of the demographics include families with kids until 48 inches tall, so the thrill rides won't be booked solid. TSMM and Soarin are family friendly, so everyone can ride them.
Before, with FP-, if I showed up at DHS at noon I wasn't riding TSMM. Soarin, during a slow time, might be available late in the day, but when I went Thanksgiving week I did the mad rush at RD to get my FP and secured a time for 1pm. Same with TSMM during August.
I'm not saying Soarin and TSMM won't book up, but it's certainly possible that there will be some available late in the day due to the rationing that FP+ provides.