It's hard for young fans to understand just how impressive this was. Dan Marino threw for 48 TD's when the next highest guy only had 36 TD's. Marino had 133% of the TD's of the next-highest guy in the entire league. Pat Mahomes threw for 50 TD's recently and was MVP of the league, and instantly hailed as potentially the greatest QB of all time. It would be like someone in that same season throwing for 67 TD's ... shattering the previous record. No disrespect to Mahomes--who is an extraordinary QB and destined for greatness IMHO--but what Dan Marino did was THE Masterclass at the position.
Marino threw for 48 TD's in an era when it was tremendously harder to throw TD's. Pass rushers made their careers off of ending QB careers .... and it was legal. When the QB dropped back, he was head hunted by every DE/OLB and blitzer. It wasn't an occasional helmet-to-helmet hit ... it was every play. They went for the big hit over the sack, because if they could knock the guy out for an entire season, they regularly got bounties and notoriety and far bigger contracts. And the QB's were not allowed to throw the ball away like they are today. Tom Brady would have retired or been out from injury by age 35, forget playing into his 40's, he'd be a broken shell long before that. The only play today that even comes close to the life of a QB in the pocket, would be Punt Returner if he was never allowed to fair catch. Guys rushing full speed with the objective of making the most violent hit possible, and the returner expected to make the magical play. If today's small QB's were asked to return 40 punts a game ... they simply wouldn't finish the season. That was how it was to play QB in Dan Marino's era. Defenders using their helmets as weapons and their bodies as missles? Yes, that was the basic plan and expectation. Helmet to helmet contact? All the time. Guys aiming to take out a QB's knees and end his career? Yes, frequently. You gotta remember ... all of this was legal back then.
DB's that were allowed to mug WR's everywhere on the field--full on double fisted jersey and shoulder grab type muggings far worse than 90% of the Pass Interference calls you see today. The reason all the WR's back then were small and all the WR's now are gargantuan targets is that the rules changed. In Marino's era, no big WR would ever create separation, because the DB would simply hop onto him like a backpack and every target he got would be contested. So, the WR's were all like Jakeem Grant, super-quick guys who sometimes got held up at the line of scrimmage, but other times were able to create separation. WR's were open a lot less back then, because of the rules. And the QB needed to have far better accuracy, because the wingspan, hands, and window those small WR's presented was far smaller than it is now. Today's bigger WR's don't need to worry about DB's really, because after the first few yards the rules require the DB's let go of the WR's ... which makes it 10,000% easier to create separation because all the DB is allowed to do today is run near you ... not really cling to you like a backpack. So, WR's don't even worry about it. The bigger WR's are strong enough to push through the "press" coverage of today--which is laughable compared to the muggings DB's used to be able to impose. The big WR's of today are playing flag football out there, and their big catch radius make them much, much, much easier to hit. I'm a DeVante Parker fan, but he wouldn't have cut it back then, because he couldn't ever create any separation. The DB's would have hopped on for the ride. Sure, Larry Fitzgerald would have still made it work, but every catch is contested, so the QB simply cannot make even the tinniest mistake or it's an INT.
These are the reasons why the great Pittsburgh Steelers DB Rod Woodson always says things like "If Dan Marino played today he would throw 70 TD's in a season." Young fans dismiss that as hyperbole ... but Woodson is being serious. Playing QB today is like hunting fish in a barrel with a fully automatic machine gun. We don't really know exactly how today's guys would have fared back then. We know the small guys would have been destroyed physically. (Marino was 6'5 and 230 lbs, listed as 6'4.5 and 225 lbs). Guys who are 5'11 and 200 lbs would be crushed back then, no matter how seldom they got hit. And to survive, they would need to throw the ball away a LOT more, which means their completion percentage would go down by somewhere in the range of 7-10%.
The game wasn't "slower" or "easier" like some naiive fans think. The game was different. More physical, more physically demanding, far more violent, and much more dangerous for QB's. Heck, most young fans only know Dieon Sanders from his commentators days. He's a "historic figure" to most of them. So many do not know that he's the fastest CB that the NFL has ever seen, running a 4.21. He could play in any system in any era and dominate. At the CB position, Sanders is the GOAT, despite not playing recently. Because while the game changed ... humans didn't evolve significantly in only a few decades.