Here is the problem with this line of thinking: You're assuming that if it was a different coaching hire that Swartz would have stayed. I don't think that's the case. Also, what's wrong with planning ahead for the contingency that Swartz leaves by hiring a defensive minded coach. Also, I'm sure you listened to the presser, Ryan stated that he does not push a system on his players. Instead, he builds a system around the roster. Isn't that what we Bills fans have wanted for years? I know I have.
In terms of offense, again according to the presser, Ryan will give his OC a long leash, something that didn't exist under the last regime. Ryan is the HC, not the DC or the OC. He knows his role and is going to fulfill it accordingly. In fact, we're probably in better hands with a defensive coach like Ryan as opposed to an offense oriented coach in terms of maintaining a good/great defense.