If by "scheme" you mean a good 3-4 defense, then a team with great players should be able to execute it (ex. Saban at Alabama, or ND last year)
If you think D is a terrible play caller (which he is), then firing him is justified. However, I don't understand those posters who constantly harp against the 3-4. I mean, the '72 dolphins used it during their undefeated season, so I highly doubt its such a terrible defensive system when properly executed. Its one thing to say D is a terrible teacher when it comes to defense, but if SFL kids can't learn a properly taught 3-4, they don't deserve to go to the NFL.