However, it is that seemingly arbitrary three-times-a-year limit applied to the People section that is most concerning. Why not four? Why not as many times as a user wants?

If it works the same way Immich does, probably because they have to retrain the models every time you turn it back on and want to aboid poeple turning it on when they need it, then off again, then back on, … Although a less shitty mehod would be to limit the amount of times you can turn it on but I guess that iterferes with their goal of harvesting your data.