Earlier quoted context omitted.
I think that long time might not be higher than an hour or two given how much heat the LEDs need to dissipate. A lot of non-technical users might get used to that brightness and then start complaining about their display dimming by itself randomly.
Apple claims on its website to support 1,000 nits sustained brightness and 1,600 nits peak brightness. If in reality they can only sustain 500 nits brightness, that's probably a disappointment to people who spent extra on the newer, fancier display in the hopes of working on the laptop outside. If they can't sustain their sustained brightness, that might even be false advertising.
(Nevertheless, still super happy w/ the computer as a whole; it's an amazing machine.)