One of the hardest roster building things to communicate is this somewhat contradictory idea. You cannot win in the modern game without a steady stream of pre-arb players, and without a couple of them being very good on top of that. But as a probability you should bet on every one of them to fall short. So you have to balance that idea that every individual prospect is a losing bet, but you have to make a lot of those bets, because not making them will lead to failure too.
The Cubs plan on being good this year, and they plan on being good next year and the year after that. Because of that, they have to be willing to deal with the imperfections that come with development in order to keep that up. Shaw might never turn the corner, and that's a risk. They've traded for 3B each of the previous two deadlines and both of them were notably worse with the Cubs(Paredes to a degree that some people thought he was not good at all), so there's risk in that avenue as well. They'll likely do something to hedge Shaw's production, but especially considering the buffer afforded by being one of the clear Top 3 offenses in the game, and that Shaw's defense/baserunning have been excellent, there is never a better opportunity for a competitive org to let someone work through those growing pains.