My understanding is Didi graded out reasonably well defensively, but he's nothing special (just better than Segura was). With the Yankees he was obviously not still himself, and they could only get him back with a QO which they decided wasn't worth it.
Not sure what offensive numbers were "ghastly," his BABIP was normal, I guess it's probably things like barrels and exit velocity?
If you look at his splits, however, he's definitely on the edge of being someone you would bench against LHP (.203 AB, .243 OBP, though still some power), and he was also not good on the road (a straight up 100 OPS+).
His success this year was largely due to being a stud with RISP but that's not necessarily a repeatable skill (his career OPS there is .769, it was 200 points higher in 2020, which is also a small sample of course).
Regardless, I would not want to put any chips on a 31 year-old guy with a surgically repaired upper body. A QO, yes. A three-year deal if you can somehow unload Segura, maybe, though we could still regret it as soon as 2022. If his bat declines, a defense-first SS with an even worse bat may provide more WAR. As it is JP already put up more WAR than Didi this season.