To be fair, Montana doesn't even call himself the greatest ever. He said the reason why he avoids calling anyone the greatest ever is because of the difficulty of comparing players from different eras, which he does have a point.
Sure, but his delivery of that argument made it seem like he was trying to keep the conversation alive where most others (including Bradshaw, Favre, Tarkenton, Warner, Young, Aikman, and many others who played in different eras) have just conceded that.
You play who you play, under whatever rules you play, and under the circumstances (team-wise, coaching-wise, free agency or not, salary cap, etc.) you play. There's no countering that argument, which of course is fair in such a subjective topic.