How long has Batman actually been in existence in Nolan's films? A year and a bit? Who's to say that escalation doesn't go one step further, and start to incorporate a little bit of the supernatural?
You're misunderstanding the entire theme of escalation and why it's important.
The entire point of escalation was that
Batman was the one who started escalating things. It was
Batman who started all the weirdness, by dressing up, by taking the fight to the criminals, by using fear as a weapon. Only afterward does the criminal element decide to step things up, by putting on makeup and costumes. It's a
response to Batman's actions.
Thus, by fighting the original criminal element - the mob and corruption - Batman has inadvertently made Gotham City
more dangerous, since he lit the spark of escalation. His goal was to clean up Gotham and make it safe for others, by fighting the mob. But he actually set Gotham back, by inspiring more craziness (both vigilantes and criminals) and more weirdness. Things didn't go to a bright calm future, they began to spiral into a nonsensical, horrific nightmare. That was what Nolan had been trying to say, regarding the theme - "Things have to get worse before they can get better."
The world made sense beforehand. It sucked, thanks to crime and corruption, but it made sense. Escalation lead to it making
no sense, it made it truly crazy - a world without rules. "Things are worse than ever."
The point then is that Bruce suffers the guilt of a failed hero, because escalation is squarely on his shoulders.
He caused it. The escalation into craziness was his responsibility. He set out to do something and it had the opposite effect, and that is the tragedy. That creates the emotional resonance.
And this is exactly why escalation can't "go one step further" and incorporate the supernatural.
Because if a baby from Krypton showed up in a farmland 30 years earlier...
...If an intergalactic police force entrusted a human with the safety of this sector and gave him the most powerful weapon in the universe...
...If a group of super-powered women exist in this world, settling on a magically hidden island...
...then the supernatural has nothing to do with the escalation caused by Batman and his actions. The weirdness
already existed in his world, regardless of what was going on in Gotham City. It's likely that the weirdness would have spread to include Gotham, if it hadn't already, whether or not Bruce Wayne ever decided to put on a mask and fight crime as a Bat.
And that takes away from the tragedy of "The Dark Knight" and the weight that Bruce must carry.
He regrets being Batman, after Rachel dies, because he knows the craziness of the Joker would never have been unleashed if he hadn't become Batman.
But if you then decide to abandon the conceit that the world isn't supernatural, if you give up the notion that there are no super-powered beings who fight crime out there, then you must allow for the fact that criminals like the Joker would have arisen even if Batman had never begun. And you take away the emotional core of "The Dark Knight" and Batman's journey.
I don't think Nolan would want to cripple the impact of his own "The Dark Knight" or make it less meaningful, for the sake of the Justice League movie.