More games = more potential for injury. People need time to rest and recover after those games, Monday & Thursday night games are already extremely demanding on their bodies.
Attempting to force teams to bench their best players for injury prevention / adding games to the schedule that will be played by the bench players will also lower the quality of pro games, and turn off even more fans to the NFL - which is already not doing great.
Additionally, nobody was talking about forcing coaches to bench players, we're talking about setting up the system such that coaches feel it's in the best interest of the team to occasionally bench (rest) players. Also, you don't send an entire team of lower quality players out at once, you cycle them through such that you're still always playing at a high level, just maybe not the peak possible (which you often aren't because of injuries anyway). There may be additional benefit to running more games when it comes to fan viewing and involvement, I'm not sure, but there's probably good data on it for many other sports.