The OP overstates the impact. We go early Dec every year and have been there during Pop Warner a couple times. I participated in Mousefest in both 2003 and 2004.
As fun as it is, Mousefest is a relatively small event. Although there may have been roughly 1000 participants overall, it is rare for a single Mousefest event to have much more than 100 participants. The MegaMouseMeet has several hundred but unless you go (at the Swan/Dolphin) you wouldn't even know it happened. The impact of Mousefest on park attendance is nil.
Although Pop Warner week has a major impact on certain resorts, the affect on park attendance in the past has been minor. Unless they change their habits, this is unlikely to change. I have yet to notice a large, identifiable Pop Warner group in the parks. Early Dec remains a time of relatively low park attendance.