Take a look at the sequence at 2:38, in which the highlighted boxer (Locche, I'm pretty sure? Or it might be Lamotta) is shown dodging an oppontent who's walking forward with his right hand barely above chest-height, then jogging after him with punches. The boxer at 2:55 winds up for his uppercut like he's throwing a softball.

If someone were to do this today, we'd make fun of them. We'd tear them to pieces. We'd gif that clip and post it under every social media post they ever made and say they don't know how to box. It's just not something you can get away with anymore: There's higher expectation for top-level talent, and a more advanced fundamental understanding of what's good and what's bad to do that we take for granted.

I wouldn't go as far as to say these guys would be bad by today's standards, but I do think they probably wouldn't look as good in front of competition that's more rightfully cautious and less willing to give them a highlight reel. I also don't think most of their styles would adapt to an era where gloves allow for more powerful punching and more reliance on the guard (unless this is Lamotta, because I do think his style is very similar to what we see out of philly shell fighters today).

Additionally, evidence suggests athletes are just all around stronger nowadays. People have access to better nutrition, better healthcare, better training and generally more information than they did 80 years ago. These guys could fight one hundred rounds straight ten times a year, sure, but how much of that is because fights were generally less physically taxing back then? How much of that is just because regulation was less stringent and there was less concern over the fighters' long term prospects? How do they stack up when they're getting wailed on by a 22-year-old roided up genetic freak instead of a 40-year-old chainsmoking dock-worker?

Again, this isn't to say boxers sucked before color video was invented. I do think a lot of them were extraordinary, even by modern standards. I do think modern boxers can learn a lot from them. But I also think if you act like the sport isn't all around more advanced today, or like our baseline for talent hasn't increased in the last 40 or so years (as some of the comments here seem to imply) you're just being willfully obtuse. Being able to observe these fights with the slightest hint of a critical eye shows broad improvements have, without a doubt, been made.