If the end result of trading for Alonso is 'when everyone is healthy and no one is being rested, either Busch or PCA are out of the lineup', I have a hard time seeing that as being a mutually exclusive situation. Part of that thinking is that you do exclude Morel from the calculus, but even as one of his longest running defensive skeptics, I think that ship has sailed for this year. The internal options have not been nearly good enough to push him to DH outside of specific matchups, and in a world where that would also push another plus hitter to the bench I think they'd be even more willing to stick it out with him.
Also, Alonso has a career 133 wRC+ and Citi Field is tied with Petco for 29th in hitter friendliness, part of the motivation for pulling the trigger is you're buying at least slightly low on what you'd expect for him rest of season.