Yeah I think that's right. They didn't trust his health. But also, if he is healthy he expects to make 120 starts, and even 100 is probably too many if you still want to develop Alfaro. Defensively they're not as far apart as we might think, given their respective ages/trajectories.
Or, alternately, they're still prepared to move on from Alfaro (or truly demote him to back-up) and there are other options out there. Realmuto is obviously better (and the buyers for him are dropping out). Grandal's a switch-hitter.