The most interesting thing that I read in that article was Yoan being quoted that he would play wherever the team wanted him to.
So, in response to the "if it isn't broken, don't fix it", the answer isn't that 3B is broken, but that 2B is. So it's not really true that it's "not broken." There's no question the Sox would be a better team with Yoan at 2B and any of those other names in the article at 3B, rather than most current 2B options who are currently free agents.
Guys move around all the time to make the overall team better. There is no reason to make an exception for Moncada.
To provide another example, if the Sox hadn't traded Tatis Jr to the Padres, and he was still on the Sox, do you think the Sox would have moved Tatis off SS or Tim Anderson?