I think it’s more that they’re cribbing from the Arnold playbook and today’s action stars have to be as much comedian as action star- look at Dwayne Johnson, Chris Hemsworth and Chris Pratt, for example.
But I think the bigger issue is that action movies have changed; the 1980s style action movies where the main characters are human and go through superhuman stuff aren’t the thing- now it’s literal superheroes for the most part, or they’re more realistic and require less action chops and more acting chops than they used to.
I think you folks might be giving Hollywood too much credit. They are just cheap and lazy and it’s a heck of a lot cheaper and less risky to trot out a proven name than it is to develop a new action star. Its the kind of short term thinking you’d expect from a bunch of publicly-held companies.
Danny Trejo might qualify as well even if he wasn’t as big name he certainly did action in films. He is two years older than Stallone and three older than Arnold. Trejo came to acting later in life than the other two, and while he’s still acting I’ve noticed of late most of his “action” is more implied than straight forward and in front of the cameras. Which these days is also true of Sly and Arnold as well. They’re in good shape for their age, but they’re all still in their mid to late 70’s at this point.
Christopher Lee was a genuine bad ass in real life as well as on the screen, but once he hit his 80’s he slowed down a lot, even if he did continue to act into his 90’s. Same for Ian McKellen - he seemed to be jumping around a lot for Lord of the Rings but found the schedule required for The Hobbit very difficult to keep up with (he’s in his 80’s, by the way).
Trejo was a genuine fighter - his entry into Hollywood was helping to coach actors in fights. It’s just that Trejo was never a martial artist - which a lot of the flying-kick style of action hero owes a debt to. Trejo was the champion boxer at San Quentin during his time there, and I’m guessing you have to be pretty bad-ass (and in good shape) to pull that off. (Lightweight and welterweight category - Trejo is not a big guy in real life (they use a lot of scully boxes when he’s filming) and someone standing 5 foot 5 is not going to be able to compete as a heavyweight). Also, that boxing “career” was back in the 1960’s, so he’s well past his prime by this point. (Still wouldn’t want to get punched by him, even if he is an old man now). Anyhow, boxing is a lot different than the gymnastic fighting the modern superhero is often shown doing.
For younger action heros - I’d suggest Chris Hemsworth, Henry Cavil, Ryan Reynolds, The Rock, and Hugh Jackman although Jackson is getting long in the tooth and apparently has danced off to do musicals (which is perfectly fine), and Tom Cruise. And probably others I haven’t named. Jackman and Cruise are getting towards the end of their “likely to be cast as action heros” part of life. I think Cruise gets away with it by being a real life “action star” what with the HALO skydives and helicopter flying in real life, but I don’t care how good a shape you’re in, you’re going to start slowing down and healing slower as you move from 50’s into 60’s, 70’s, and later.
To some extent, on average people these days tend to be healthier and less aged than in, say, the first half of the 20th Century. We tend to look younger than our parents, and certainly younger than our grandparents, of the same age. Fewer childhood diseases to endure (except for the children of anti-vaxxers). Better nutrition. A potential for better exercise. Better treatments/rehab when we do get injured. Probably does extend the shelf life of many actors.
As opposed to playing the Terminator, Rocky, or James Bond (as three random examples).
One major reason so many older actors are still making action movies is that they’re much less likely to smoke, less likely to drink heavily, and more likely to eat a vegetable every now and then. That’s going to keep you younger looking and more active a lot longer than the gin-soaked chimneys of previous generations.