How much more money does it really make them? They are already selling out preseason games at regular prices.
The Packers are one of the few teams to sell out preseason games. Two more reg. season games for all teams will mean a ton more fans for the season.
Will it make them more money with TV contracts?
Don't know, but I'm guessing yes.
Also, while an extra 2 games could lead to more injuries, it also gives teams the ability to withstand an injury early in the season. Say a star player gets hurt in week 3 and is out for 6 weeks. They're season is basically done. If the same player gets hurt in week three with an 18 game schedule the team now has 2 extra games to try to get back into it.
I think with a 18 game schedule, you'll see more late season injuries than ever before, since the players will be going full effort for two extra weeks. People think that isn't a big deal, but it is.
This is a bad idea all around.