It's

when you think about it. I finished 103 of my AG last Princess race (out of about 1600 runners), and my time isn't even good enough for the POT. I know not everyone races the races at Disney but still.
They've used several different cutoffs since I've been around (2014). Based on my notes, they used:
2014-2017 - 5 hour marathon (or equivalent)
2017-2020 - 5:30 hr marathon (or equivalent)
2021-2022 - 4 hour marathon (or equivalent)
2022-2023 - 4:30 hour marathon
ETA (3/10/2020): After contacting runDisney, I have received confirmation that the 2:30 HM POT cutoff is the new standard moving forward starting with Wine and Dine 2020. They provided me with the conversions necessary for each of the submittable distances as seen in the new updated chart at the beginning.
ETA (4/2/2020): runDisney has updated their website to show the new POT cutoff for the marathon distance is 5 hours. A new updated chart as been added and will be updated with official cutoffs when that information is available.
ETA (6/30/2021): With the announcement of the 2021/2022 runDisney calendar came a new HM POT cutoff for Wine and Dine 2021 at 2:00 hrs. There is nothing listed for the Marathon at this time. I've included a new theoretical POT cutoff sheet above until we learn more.
ETA (7/19/2021): Confirmed 2021 Marathon Weekend cutoff of 4 hours or less per runDisney's website.
ETA (4/9/2022): Confirmed cutoffs for HM events moved to 2:15 POT equivalent and for M/Goofy/Dopey to 4:30 POT equivalent.
About 10-12% of the field have a POT under 4 hours, about 31% of the field has a POT under 5:30 hours. So if you set the cutoff at 5.5 hrs, then about 45% of the field has a POT. For the non-POT selections, about 30% of the field selects the fastest non-POT. So if you set it at 5:30-6:00, then 30% of the field selects that. The remaining 25-30% of the field is heavily weighted towards the last corral (6:30-7:00) then the other choice (6:30-7:00) with a split of 7% vs 22%. All my data from the last ten-ish years shows that the population that makes up a runDisney race is pretty similar year to year.
Yeah, a less strict POT might also relax it for some in the corrals.
Ultimately, the goal is to get 15,000 runners (or more or less) across the start line in 45-60 min. So the rate of release is the same whether there is a POT cutoff or not. The 10,000th runner crosses the line (or better yet the 66 percentile runner) at the same time regardless. What having the POT cutoff does do is reward those who do have it with a proper seeding. The issue they ran into when they had a set number of runners per corral (like 2000) was that so many people chose the exact same estimate time (back then you manually entered 5:30:00 vs 5:30:01 instead of the drop down choice) that it created multiple corrals with 5:30:00 because people figured out the system. Then "Susan" and "Joey" weren't in the same assigned 5:30:00 corral and emailed/complained to runDisney because they had more than 2000 people submit the same exact time. They mostly fixed that with doing away with set number of runners per corral and instead do larger corrals with mini-waves. This combines all those 5:30:00 runners into one corral, but then splits them up within the corral by staggering the release rate (mini wave).
So what increasing the POT cap would do, is it would create a less stressful environment for the POT submitters. Then the other 55% (or a lower number if the cap were moved to say 6:00 or 6:30) would be still starting at the 45% mark (so in a 60 min release they would release at 27 min past gun time). So the amount of people around you would remain similar if you're the 10,000th runner. But what it can change is that someone with a potential POT of 5:29 wouldn't be standing next to someone who doesn't have a POT. So instead of being the 10,000th runner, now the 5:29 runner has been moved up to the 6750th runner (based on the % of runners in a 15,000 field that would have a sub-5:29 POT). Their experience will feel different from that standpoint. A different runner will become the 10,000th runner and their experience will remain the same as any other 10,000th runner.
The biggest issue from runDisney's standpoint on changing the cap to 6:00 or 6:30 is whether they should verify each POT. Back before they moved it down to 4 hours, the belief was that they couldn't possibly be verifying them all. It was cumbersome. Chicago marathon, sure. But runDisney with multiple weekends and multiple races per weekend with POT submissions makes the number staggering in comparison. So the conventional wisdom is that maybe they were doing some spot checking. After moving to 4 hours, it became much clearer based on anecdotal evidence that they were verifying times. So if they move the cap to 6:00 or 6:30, does runDisney verify all those POTs. From their standpoint, is it worth it? It's worth it to the end user (us the runner), but what does runDisney gain from all that extra work. Their races mostly sell out at the price they set as is stands now. What's the extra effort gain them? So in my view, you go back to verifying by spot check. But if it were me, and I found someone outright lying on their submission (hence not a mistake) I'd ban them for a year or more. Don't waste my time with a knowingly false submission. But again, if I look at the numbers from the last ten years, the data suggests to me that people are willing to put a "fake" estimate, but they're not willing to put a fake POT. There's always a bulge of runners at the first non-POT no matter where they've set it.