I would liken it more to a car battery's death.
Superman was a living solar battery. He expended all of his stored reserves defeating Doomsday. Powerless, he died.
The regen chamber jumpstarted Superman by exposing his cells to solar radiation.
How long would it have taken if he hadn't been placed in the regen chamber? No idea. We may find out, since there's been a death on New Krypton. Or it's possible that the idea that Kryptonians will regenerate if exposed to yellow solar radiation will go away as a post-Infinite Crisis
And I'm not seeing the "retcon," Christopher. The regen chamber was from the original Return of Superman
storyline. What it did was explained in that storyline. It didn't magically appear in a later story. Maybe Nekron's statement that he prevented permadeaths in Blackest Night
#5 counts as a retcon, but it doesn't retroactively rewrite the past. You could argue that it's a retcon in the sense that it's a continuity implant, but by that argument a whole lot
of stories count as retcons, like your own Ex Machina
or The Buried Age
, and I don't think anyone, least of all you, would ascribe the term "retcon" to describe them.