Your list includes nobody recruited after 2011 aside from Thompson. The 2012 class-on is when this slide started. Probably not coincidentally, that's when most of Matta's better assistants left. And ultimately, isn't it the coach's job to motivate the players to put in the work? One or two guys coming off as lazy, and/or having flippant attitudes towards the game can be labeled as "bad apples", an entire roster? Not so much. Something stinks beyond just lazy/uncommitted players.
Name one player from the 2012 class onward that you would say "developed" here. Thompson is arguable; he had his moments, but was also prone to some horrible mistakes, mostly of the foul variety. It's worth noting he was a transfer. D'Angelo Russell was an elite talent that played like one. I suppose you could include him, but I think he'd have succeeded pretty much anywhere.
Amir Williams, Sam Thompson, Shannon Scott, Marc Loving, Kam Williams, Jae'Sean Tate...all of these guys received significant playing time over multiple years, and none made or have made significant leaps in their game...and I'm not talking superficial statistical bumps due to increased minutes. tOSU has had one player that came through this program since 2012 playing in the NBA (Russell). ONE.
I think Matta's biggest issue has been questionable assistant hires. As his purported health issues started, one would assume he would have to lean more on his staff. Clearly the staff isn't getting it done. Ultimately, that falls on Matta.