Likewise, you're free to pointlessly handicap entire domains because you inexplicably think the Dark Lord must have the biggest CR in their Domain. I'm just saying your assumption doesn't make a lick of sense.
Most important, yes. But not necessarily the most powerful by CR. There are more forms of power than are reflected in hit points, DPR, and CR. Political power, control of resources, contacts, alliances, etc. How does immortality affect a creature's CR? Or the ability to block or allow passage through the Mists surrounding the Dark Lord's Domain? And as you quoted, the Dark Lords are immortal. How does that affect their CR?
I’m objecting to the notion that a Dark Lord's CR is reflective of how difficult the Domain is as a whole. The Dark Lord’s CR is not the difficulty of the Domain. It’s not a suggested level limit. This is literally never once suggested in the book. That's an assumption you are making. I'm just pointing out that it's your assumption and questioning why you'd make it.
And the book agrees with me: "Darklords range from monstrous tyrants to depraved individuals who otherwise appear as ordinary people. A Darklord’s position and powers stem from their deeds, not their game statistics. While some Darklords are deadly or possess foul gifts from the Dark Powers, many pose little threat in battle. Their menace is the result of capabilities beyond physical prowess."
The CR of a given Dark Lord is not an indicator of the "difficulty" of their Domain, nor is it a "level limit" for the creatures contained within.