I'm thrilled (well, really happy anyway) with the way Castro has been playing for roughly the last month. He's been a real asset to the team during this period, and the way he's going he could be a real key in the playoffs.
But that said, now that he's presumably improved his trade value as well, I do think it's likely he will be traded.
Assuming the Cubs want to upgrade the team through trading, and that Castro is valued by other teams, he seems to me the natural guy to go. So much of his game just doesn't fit with the profile the Cubs want in their players. He's really not a very heads up guy, either at the plate or in the field, he does as well as he does because of his natural gifts.
Not only do the Cubs have two better middle infielders in Russell and Baez (defensively for sure, and very possibly offensively), they have Gleyber Torres in the pipeline. Yeah, he's only 18, but he is performing well above his age, and he may well be at AA sometime next season if he keeps it up.