Where are the studies showing that turning two pre-season games into games that count will appreciably increase the actual number of injuries?
The CFL, and they're not playing touch football, have been playing 18 games with a smaller roster for decades.
As a former pro athlete in a "crash" team sport, I know, as all players do, that the most physically dangerous time is training camp and exhibition games where idiots are throwing their bodies around recklessly in the hopes of making an impression.
It's impossible to create a study that shows 2 extra games will definitely cause more injuries because they haven't tried it out yet; however, if you look at the number of injuries this year and the previous years, it seems all too likely. There were actually a record number of players on IR this year. Multiple owners, including Bill Ford, have come out and stated that they are against the 18 game season because of injuries. I have links to back this up, but I can't post them due to my total post count (it's too low). Let me know if you want me to PM it to you.
Playing in the NFL is like fighting against Tyson in his prime. Every time the players play another game, it's a gamble: they take a serious chance of injuring themselves. Adding two extra games just increases the likelihood that they will be seriously injured and unable to continue their career.
To your point about the CFL...the CFL is not the NFL, and it will never be. Yes, they aren't playing touch football, but it just doesn't compare to the physical abuse your body takes when playing in the NFL.