Even if we say that the team has no intention of holding onto Tucker (which whatever, I have no interest in arguing that) there's really not room for all four of the Iowa bats who are currently pounding on the door.
After this season you're replacing Tucker and Turner, who will combine for 800-900 PAs this season. So that's two kids playing not quite full time. Maybe you keep a third for injury and redundancy? That still leaves a guy who you have no room for on next year's squad.
More room opens up after next season, but there's not a realistic way to hold out for that. At least three of those guys are MLB ready *right now*, and the fourth isn't super far off. You can't just hold their development in stasis and wait 20 months to give them their first extended shot at the big leagues.
This week was the perfect time to deal at least one. It's not the end of the world to make a deal this winter, but it comes at the expense of not improving a team that has a decent argument for being the best in the NL.