I wouldn't exactly say that Shea passed along a D2 roster (close .. but ..). If you averaged it out - it probably would've been a lot closer to an underwhelming Group of Five team meets average FCS team. I mean, look at UConn (particularly under Diaco). That's FBS, as hard as it is to believe. Not Alabama but FBS, nonetheless. I'd say that Schiano inherited some decent players - like the ones mentioned above. But, with Shea's poor coaching and - most importantly - lack of player development ok recruits/players become nothing. And, it showed.
In contrast, I'm fairly amazed that Schiano has already improved the Defense as much as he has. And, I don't just mean with regards to transfers. Some of the guys on the field (starters, even) are the same ones who have been in the program now for 3-4 years now. They looked awful under Ash. But, in less than a year under Schiano look good. The DBs know how to turn their head around, the LBs know how to tackle, and the DL can (and do) actually get pressure on the QB.