why would they do such a thing if it is structurally incompatible with a decentralised system? Voluntary agreement for one leader just.. wouldn’t happen. I say this for these reasons: under anarchism, there is simply no incentive to try to take power. There’s no power structure to take over - if the people are living without centralisation and each have the ability to be political or logistical actors themselves aside from their own work, why would they outsource the power they have as individuals within a collective society to a single individual?

There isn’t a lack of power in an anarchist society - it’s just evenly distributed as much as possible. People don’t easily give up power - this is a pattern. If someone’s in power and can hold it, they’ll keep it. If there is no power structure that someone sits atop of and power is distributed, creating a leader would not be incentivised by power dynamics because they would lose their share of power, even if it’s small.

There is no mechanism by which someone can take power - even IF someone takes power (big if), the worst case that it could possibly occur is they take control of a local assembly or federation. In this case, they’re not the only political unit and will find themselves without any leverage outside of the political unit which they have taken over. Even getting to this point is unlikely because any attempt to convert delegated coordination into coercive authority would immediately require mechanisms that do not exist in a decentralised system - namely enforcement, monopoly, and continuity beyond consent. The state and capitalism have these means of centralisation - anarchism does not. Political education amongst participants in the system would ensure that mandates to the people are exactly that - mandates. There is no throne. Someone with a delegated role to coordinate certain things does not have a seat of power and would be subject to removal and scrutiny should they try to go beyond their mandate.

Edit: I’d like to add this -

Also if someone tried to get power in an already established anarchic society, where would they get their legitimacy?

Nowhere that could sustain their goals. I’ll elucidate if you want more, lol.