The difference between a 1B and COFer is half a win a year, and that's assuming he is similarly below average defensively on the dirt. That adds up, especially over a long term deal, which is why I don't expect Juan would make the move before getting paid.
But for a team, that half a win can be more than made up for by the specifics of their roster. The Cubs have corner outfielders coming out the ears, so Soto staying in the OF rather than shifting to 1B would 1000% be about deference to the desires of a superstar, and not about the pure baseball decision-making.