It's a horrible thing to sit in judgement of. Naturally I've had cases where someone has has claimed "but ma mental health!", and I've felt they were shamelessly disrespectful for people with real struggles to try to get something they wanted. (This is why I must an unvaccinated indoor pint, take my dog on a plane, never be subject to any criticism no matter how poor my own behaviour is, etc.) Biles doesn't strike me that way, but even for others I'm leerier of, well, who's to judge? One will never know, short of being in the other person's head, or the other person's therapist's chair. Maybe not even then.
They also have alternates so she was replaced. In fact her replacement won the individual all around.
Not exactly how it worked in either event, as I understand it. The team format is play four, carry three scores. No replacements allowed, so that meant that the US had no 'discard' available, while the other teams did. Biles's score in the vault did stlll count, so not a gimme whether a hypothetical fifth gymnast would have given the US a better outcome. On the face of it it looks like they'd have done at least as well, as the two other US gymnasts
not in the team event were higher placed in the individual all-around, so I assume the logic of the team selection was to use the discards to their benefit with competitors who're strong on some apparatuses and weaker on others. It would of course have given that other gymnast a medal (in all likelihood, of some kind), rather then Biles.
The individual finals are obviously, well, individual, though there is a two-per-country limit from the qualifying phase. (It is NationalismBall, after all!) Sunisa Lee was the second-highest-placed US gymnast, so would have been grand regardless. Jade Carey was the third-placed American, and was allowed to compete after Biles's withdrawal, so loosely speaking you could say she was the "alternate".