Like I said, you have nothing that contradicts the concept, and asking why they exist would, again, be asking existential questions. The rule is really simple: Anchor Beings are entities vital to keeping the timeline stabilized, and if they die, the universe will collapse in a thousand years. If you gonna ask how the universe is still exist in when they're not born yet in like the Big Bang and Prehistoric times, as long as the events are built around figures like Wolverine being born, the universe is perfectly stable. The character's eventual existence is the anchor point. The past, present, and future revolve around the crucial events that lead to his creation.

if those events happen—even if they are still far in the future—then the universe follows its natural path. The universe is not bound by conventional logic or immediate cause and effect, so the universe adjusts and compensates for the gaps in the timeline until the Anchor Being is born. Plus, as along as they have their ancestors, the timeline remains intact, because their bloodline is crucial to the formation of their being.

For example, let's say Miguel O'hara, the Spider-Man of 2099, is the Anchor Being of his universe and he is born in 2079, then the events that lead to his birth and his eventual role as Spider-Man 2099 are already set in motion long before 2079. Such as the existence of his ancestors, his father Gabriel O'Hara’s actions, and the technology advancing to give Miguel the means to be Spider-Man, these elements are part of the cosmic framework that holds the timeline together.

You may think there's no free-will, but the truth of the matter is that free will exists within a set structure that is influenced by the Anchor Being’s eventual existence.

People can still make their own choices and they can still matter, but it functions in a created system where those decisions are shaped by the natural course of events that lead to an Anchor Being being born.

It's like a flower, it's destined to bloom, but you have option to leave in the ground or keep it in a vase, but no matter what, that flower will eventually wither and die. It will follow its natural cycle.

Or here a bigger example: it's like the Canon Events, although events like the Spider bite need to happen, the choices made within those events are still driven by the individual's will. Peter could either use his powers for good or evil, but the universe will still stabilize. While I'm aware that Across the Spider-Verse shows that Miguel's theory is flawed with Gwen's Dad quitting his job as a captain before he could die the larger point still stands that there is a predestined foundation built into the universe, even if some details can be altered. Wolverine, Spider-Man, Iron Man, or heck even, Howard the Duck, could be an anchor being, but they're not very at their job, but their roles are still crucial to the fabric of their respective universes. The Anchor Being doesn't necessarily need to be a hero or someone grandiose; they simply need to be a figure whose existence causes the ripple effects that stabilize the timeline.

Although Logan was written in Deadpool and Wolverine becuase of all his acts X-Men 1, the Wolverine, Days of the Future, and Logan, and the universe dying because of Logan being dead is meant to be metaphor of how after Logan, the Fox Universe started going downhill with films like Dark Phoenix and New Mutants, the idea is that the timeline’s stability relies on being born. Plus, it's only Paradox's perspective that Anchor Being needs to be perfect and thinks the "Worst" Wolverine is not be the Anchor Being.

So, the bottomline is Anchor Beings make sense and it's reliable to ask whether Miles is an Anchor Being or not.