Just the numbers make it harder now. In 1959, there were twelve NFL teams (AFL didn't exist) and one playoff game. In 1972, there were 26 total teams. Brady's run has been during an era of 32 NFL teams. With 12 teams, a team randomly has an 8.3% chance of going to the championship. With 26, the chance falls to 3.8% and with 32, 3.1%. The Patriots are the only dynasty during the 32 team era (Panthers, Jaguars, Ravens, and Texans were all added.) There's one champion each year, so the more teams, the more difficult it is to win.
 
	As to your other argument, I have to disagree. The NFL has made it progressively harder to be successful year after year. What makes Brady the GOAT is the number of years he's been great. Most other QBs that have played to 40 spent a lot of their careers as backups. Brady's been a starter since his second year in the league. He's won his five Super Bowls over a fifteen year period. Montana is second in this, winning his four Super Bowls over a nine year period. Bradshaw won his four over a six year period.