Sure, that could have happened. Do we fire someone based on what could happen? Evidently we do. After Fulmer's first 5 win season in 2005, he followed up with 9 wins in 2006, then 10 wins in 2007, then back to 5 wins in 2008.
In 2008, he had a new offensive coordinator with an offense style that most players knew nothing about. It could be argued he didn't have the personnel to fit this style of offense. The best fullback he had, which figured highly in Clawson's offense, tore his ACL. I'm not making excuses but you got to admit there were plenty of things to overcome.
Fulmer had proven before his teams could bounce back from a 5 win season. Maybe they would have again. Maybe they wouldn't. I think he deserved another year.