It’s also why, in ROTJ, he loses to Luke due to internal conflict, and when Anakin finally redeems himself, he succumbs not only to the injuries from the lightning disabling the suit, but all of them.
This ties into his arc of accepting the natural course of life. No longer using the dark side, a twisted perversion of nature.
I want to clarify: the intent isn’t that Anakin/Vader can’t live without the dark side after Mustafar. That takes too much agency.
A big point of the 2017 comic run, which I think matches the OT, is that Vader chooses the dark side during his reign as a Sith lord, due to ego and power.
The reason he dies after letting go of the dark side and killing Palpatine in ROTJ is because without the dark side’s power, he’s too vulnerable to withstand the lightning and damage to the suit.
But he wouldn’t drop dead if he had let go of it prior. He’d go through a weakened withdrawal period for a bit, like a drug addict would, but he wouldn’t die. He chose not to let go of the dark side. This also means that he knows that saving Luke and turning back to the light will kill him due to the Emperor’s lightning and withdrawal, which is the point of the scene in ROTJ.
I don’t want him using he dark side for self-preservation, like he can’t live without it. I feel it victimizes him too much.