We debunked this argument about 30 posts ago. With such a large disparity in the number of home games, it is nearly impossible to predict what average attendance would be if the NFL played way more home games, or if MLB played way less home games.
As I asked before (and no one answered), why doesn't the NFL play more games per week? There is nothing stopping them from making the necessary roster changes to do that. I suspect they don't do it because there wouldn't be enough fan support to make money at it.
If the NFL played too many more games in the season, there would be a lot more franchise QBs hurt throughout the year. This is what stops them from expanding the season. Many even propose limiting the preseason games to prevent injuries. Football is a contact sport and guys can gety hurt a lot easier than in baseball. (Not to say that baseball players don't get hurt.)
Also, the NFL has games on Monday, Thursday, and Saturday nights and they all have huge attendence. College football is on several times a week as well and has good ratings most every day.
If the NFL did add say, 4 more games per year, I think the average attendence would be the same. People with season tickets would be there every weekend.
I think baseball is more of a polarizing sport than football. I will watch any football game on TV, college or pro. I think there are a lot of people out there that would say the same. Most baseball fans are mostly fans of just one team and follow them religiously.
One thing that I think is tellis is that there are more mlb teams that have attendance problems and can't fill their stadium. You don't see that as much in the NFL, probably because there is more parity and teams (except for the Lions) can get good again quickly.
A previous poster made a point about the salary disparity in the MLB. That is a huge problem for the league. It means that the rich, more often than not, will continue to get richer. The NFL has limits so that one team cannot just buy the best players every year ala the Yankees.
Personally, I love both football and baseball, but football is more popular, at least to me, and it really is an apples vs oranges comparison.