You understand that the rules are vastly different now than in the 80's, right?
The rules have been changed over and over to favor the offenses and make the passing game easier.
If you look at the list of top passers of all time in terms of passing yardage, most of the Top 15 are from the past decade. God didn't just start making better QBs.
While everyone is anointing Mahomes the greatest of all time already, the guy plays a game in which he can't be touched. QBs in the past had no such pampering.
Eli, Rivers, Ryan....are these guys Top QBs of all time? Hell, no. But stats can lead some to think so.