Kelsey Plum got off to a lackluster start to her WNBA career, then she took a jump, in her third season. Then she got injured, and missed last season, and now she's working her way back. As far as whether she chose to be play 3x3, or was she not good enough for 5x5, I think that you're starting from a false premise: it's not either/or. Unlike with 5x5, there's an actual ranking system for 3x3, and you have to be ranked, in order to be eligible for a team, so you definitely have to choose to play 3x3. But that doesn't preclude her from playing 5x5, in the future.
The other thing that needs to be acknowledged is that the women's senior team is kind of an insular community. You spoke, during the Olympics, about how much you wished that the men's team had more continuity, but an argument could be made that the women's team overcorrects, in the other direction (at least, IMO), to the point of keeping players around too long. We've already lost the best point guard in the world to Hungary (Courtney Vandersloot, in case you didn't know), a country that isn't even ranked, in part because she got sick of waiting on Sue Bird to be ready to "pass the torch" on the senior team. Becky Hammon got herself a Russian passport, and played for their national team, for the same reason.
Point being, just because a player isn't on the 5x5 team. doesn't mean that they're not good enough: the team that just won the gold medal didn't have the twelve-best American-born players on it, and that wasn't because they had players turning them down, either. Have you heard anything about the Ogwumike controversy?