Battlefield already has a Free2Play version, and it sucks:
http://battlefield.play4free.com/en/
Call of Duty and Battlefield will remain successful as long as the companies can convince the audience that they are constantly improving with each version. The problem is convincing the next generation that COD and Battlefield are the franchises to get into, as newer series will pop up that bleed potential customers (Titanfall is a perfect example here) a young teenager without any previous gaming experience getting an Xbox One or PS4 for Christmas will be looking for a shiny new FPS to play, and which will they more likely begin with, the 6th sequel to a game, or the first instalment in a whole new promising franchise?
That's how the playerbase for COD and Battlefield will fall: by the next generation passing over their sequels for new franchises, leaving the COD/BF playerbase with an aging, dwindling population that changes it's formula to try and fit the new gimmicks of the new franchises, and ends up failing to gain a new audience and shunning the old audience.
That's what happened to Medal of Honor.