what proof we have 70's 80's were better eras with better fighters?

Joe Louis fought 69 times. 40 times his opponents were over the 200 lb mark. The rest of the time they were usually just below the Cruiserweight limit of 200 lbs. None of them weighed less than 180 lbs. Your statement is factually incorrect.

I'm actually pretty surprised that some of these weights are so low for how big the guys are. You've got 6'6" guys weighing in at about 207 lbs without an ounce of fat on them. You look at their photos and they are ripped and chiseled out of marble because they had to be in great shape to go fifteen rounds. Today they'd waddle into the ring weighing 280 lbs with their guts hanging over their shorts.

 

they were more offensive than defense unlike at present when there are more defensive fighters just pot shotting to win like the amateurs. life was harder before so they worked hard to stay fit and not rely on cheating thru PEDs just to be in top shape. boxing today is more focused on money and undefeated records rather than making legendary fights so there are a lot of cherrypicking and avoiding potential legacy matches.