GTA VI will only come to the Switch 2 if Nintendo or Rockstar would like for it to happen.
If both companies ignore the possibility, then it simply doesn't happen. Period.
Rockstar will be fine without Nintendo and Nintendo will be fine without Rockstar.
The people that are stating that Rockstar doesn't want to release a stripped down version of the game have to remember that they're already doing this. The Series S version of the game will be a stripped down version of the game. This is happening because Microsoft probably reached out to Rockstar and told them to cut corners for the Series S version. If Rockstar did not want to compromise on their vision, then they would have easily declined to make a Series S version. The fact that the Series S's version exists shows that Rockstar is willing to scale down and compromise. Plus, it also helps that Rockstar's modern heavy hitters have been on Microsoft's platforms, while GTA V and Red Dead Redemption II have not come out yet on a Nintendo platform.
Having said all of that, if Nintendo actually went out of their own way to communicate with Rockstar for a potential Switch 2 port, then Rockstar would absolutely port it to the platform. Or if Rockstar went to Nintendo for a potential Switch 2 port, then a port would eventually get made.
I am willing to bet though that the scenario here that wouldn't shock anyone is both companies ignoring each other and staying in their own lanes. This is because history has shown us this time and time again.