I imagine they did prepare for the possibility of trading, benching or releasing him had Miller been ready to go. But between that and Sosa having an off year and Kemp/Reyes flopping, they didn't really have any options once Bichette bailed on them.
Now, however, they do. If he has one good game every two weeks and gets three or four good pinch-hits coming off the bench he'd be worth every penny. Just not worth it as a 1B who keeps De La Cruz (even more than Hill, who at least can still platoon in CF) on the bench and Harper in the OF.
I think the team is just tired (and old). It's completely reasonable for Sanchez, who pitched 200+ IP for the first time last year, and Wheeler, given his age and injury, to hit a wall for a while. Last year Luzardo did. And the big 3 are in fact our older players besides JT. The heat's getting to them, Harper and Schwarber both have known ailments, Turner's Turner, and then of course there's the pen. Doesn't mean they can't bounce back, but being out of gas is also a real possibility.