Proof of work has always had an economic flaw that you could theoretically temporarily rent enough mining power to perform double spends of more value than the cost of renting those devices.

But this attack has never been performed because the reality of all these cryptocurrencies is that the security depends only relatively weakly on proof of work. Instead it relies on trust between the main stakeholders: miners, big nodes and developers. This is just like any other human organisation. That trust is only reinforced by proof of work, making it easier for new parties to become trusted.