I fully agree with you because the quality of player has gone up so much. I think if you took all the best players from the 70s and put them together they don't beat the Panthers more than like 3 times out of 10. Sure some would still play great, but like you said defense is just so much bigger and faster now. The linemen are so much more athletic now.
I don't think that's as true of the 80s on. I think some of the great teams from the 80s/90s would still get wins in the modern game because that's when the modern athlete type started showing up. Like Reggie White would still eat today. He may not be as amazing, but he'd still do well. Barry Sanders would still be great today. Jerry Rice would still be great today. etc. Anthony Munoz would still be a quality tackle. I dont' think Roosevelt Brown would be an elite OT in the modern game palying at 6'3 in the mid 200s. Even with modern lifting getting him to 300, but that would make him basically Jason Kelce. The only elite guys from that era who I think would struggle without some heavy changes are the big fatties. You just don't see strong unathletic fatties in the NFL anymore because speed matters. Like the Guards and Centers now are undersized tackles. Like they have mass but they're not Nate Newton or Tony Siragusa.