There is no backwards causality in the implication that building or preserving muscle that would otherwise be lost can prevent or delay frailty, nor is it backwards to imply that frailty can lead to death (from falls, disease, etc.). I really don't understand what you're trying to say.

"Muscle" isn't an intervention.

"Building muscle" is an intervention and has extremely well-documented mechanisms that have a causal role in improving health.

Muscle actually is an intervention for one of the biggest risk factors in metabolic syndrome - insulin resistance.

Muscles use glycogen. They use more glycogen when you're doing the sort of thing that builds muscle, so of course it's even better there, but someone who just genetically puts on more muscle at the same level of activity as someone who puts on less will still use more blood sugar for their muscles, and thus be less likely to increase their insulin resistance.

Ok how do you "intervene" with muscle? Do you surgically transplant it?

It's a thing that exists. Genetic differences are not interventions. Those are also simply things that exist.

An intervention is a change in treatment or behavior that induces a change with effects we want.

Differences between individuals are not interventions.

The intervention of interest is building muscle through strength training.

There's a significant number of posts in this larger thread saying that it is basically just the benefits of exercise that is providing the positive health outcome and not just having the muscle, and I interpreted intervention in a broader manner based on that context.

My point is simply that muscle in and of itself has positive benefits, even if you didn't need to do another healthy activity to maintain or gain it.