Surely the more obvious answer is that Hayden and Langer (and Sehwag etc) played during a notoriously easy era for batting while the past decade has been notoriously hard?
Seems to me that the success of bludgeoners like Sehwag and Hayden at test match opening pairs more neatly with a lack of sideways movement during their era than with a lack of machismo in modern opening… machismo as a method tends to go out the window when she’s seaming all over the place.
I guess I don't mean machismo so much in terms of slapping the bowling around...moreso that it was an admired spot to bat in. If you did that spot well, whether you were Hayden or Sunny Gavaskar, you were as as big a dog as it got on the international stage. You crafted your game mainly around being good at that job (and maybe you changed it a bit for ODIs). You played more 4-day first class cricket, too.
Is the bowling harder now? Yeah, potentially it is. That's got to explain some of it.
But to me, it's the fact that the Haydens, the Sehwags, the Gilchrists, anyone else who successfully wacked it - they set their games up technically coming through the grades. They built a power game upon good technique in age group, then domestic FC cricket. England are a prime example of trying to build a power game without the necessary technique to have it translate into Test cricket. Roy, Buttler, Morgan etc just can't cut it because their technical base is sub-par - and unless England address that throughout their whole system, they may as well say to hell with anything involving a red ball at Test level.
What I'm trying to say is that the quality of Test match opening batting in particular I think will continue to fall, because you don't get the cream of the crop. If Matt Hayden was 18 now, he'd be wacking **** out of it, chewing gum and walking down at Big Bash bowlers trying to pick up an IPL contract - not amassing thousands of Sheffield Shield runs. The top echelon of quality coming through now will most likely want to bat 4-5 in Tests and play all 3 forms + beloved by IPL teams, ala AB de Villiers.