I honestly don't think Quinn would have been a good choice for them.
He's only had one year as a DC at this level (and even then his interviews were not impressive enough to be Carroll's first choice, he chose Bradley over Quinn during his first season).
He inherited a Defense that was already the #1 scoring D in the NFL. He just helped them to get even better and more consistent. But it's not like he took some scrubs and made them amazing. Plus, he had Avril and Bennett to work with where Bradley did not.
And it was Carroll's Defense that Quinn gave input into. It's not like we have an Offensive-minded Head Coach and Quinn ran the show on Defense.
Don't get me wrong, I think Quinn is a great coordinator and an improvement over Bradley. I'm extremely happy he is staying another year. As a DC, I wouldn't want anyone else.
But I just don't think he's ready to be a HEAD COACH. Especially one in a situation where the team is desperate and needs some stability. He needs some more time, IMO.