The math can tell us how many people it would take to exhaust the supply of FPs. But math does not tell us how many guests will be organized enough to make their reservations 60 days out, 30 days out, or at all. It also doesn't tell us how many guests would not select TSMM as their tier 1 FP at DHS, either because they prefer RNRC or one of the other choices, they have done TSMM before and aren't interested in doing it again, or just because they don't know any better about what FPs are more useful.
The math also doesn't tell us whether offsite guests will be given the opportunity to book FP+ as early as onsite guests. My guess is that if Disney detects that FPs for popular attractions are going to sell out early, they will give onsite guests the first crack at them.
The math also doesn't tell us how the vaunted Disney marketing machine is going to try to push FP+ and how uninformed guests will respond to those efforts. Unless they give out the most valuable FPs to guests on the first day of their 60 day window without requiring any action on the guests' part, the uninformed are not likely to be snatching them up at the first possible moment. If they take the easy way out and go with the "fast picks", they probably won't be getting the most coveted FPs either.
There are a lot of Disney policy decisions and human behavior factors at play here in addition to the math. I do not think that sell outs of FP+ reservations are going to happen within hours after they become available except maybe for the very busiest days, and maybe not even then.
Either way, until shown otherwise, those 60 day sell outs are far enough away that I'm not going to worry about them until they become a reality. In the meantime, when I have a trip planned, I will do my best to make FP+ reservations as early as I can just to get a stake in the ground. I have to do the same thing to get flights at the best fares possible when we travel on popular dates.