So lets say bitcoin is eventually gonna mean the downfall of the bank
as a wallet is basically a bank. You can store money and make transactions all freely and
autonomously.
One interesting contrast between banks and bitcoin is the paradigm shift from "trust based" to "trustless systems".

As you said your wallet functions like a bank. This has potential to eliminate jobs in the banking industry which could streamline operations, increasing cost effectiveness and efficiency. Similar to walmart and amazon, blockchain based financial systems could cut middle men (workers) out of the equation, reducing costs, increasing profits. It may have potential to eliminate banks entirely.
In the real world, none of this may happen. Banks might cut jobs and layoff workers after shifting to blockchain based systems that's one thing which could be inevitable, under a transition from trust to trustless financial systems potentially reducing workforces and killing jobs. As for the rest, who knows.
So when that happens and the banks and the government can't cooperate anymore. And the majority will be payed by bitcoin and wouldn't that
force governments to also start using bitcoin (or any other crypto currency but it wouldn't really matter to much as bitcoin is like the backbone of every coin) which would naturally mean that their own currency will lose its value by the amount that the government choses to use the bitcoins instead. And lets say one day theres not much fiat left ... how would the value of a bitcoin measured? in Eur USD Yen?
Or will it ever even come to such a scenario?
Just really wondering

Many central banks have expressed intent to issue their own crypto currency. Even socialist(possibly communist) venezuela plans to denominate its oil transactions in a crypto currency which they announced they will issue at a later date. That would appear to be the path they will pursue.
It could be a bad idea for them to follow this road. One of the things which keeps them centralized is all of them relying on the same financial network (SWIFT). Switching to independent networks where they issue competiting crypto currencies utilizing different blockchains, may produce greater competition between central banks which could fracture and destroy the unified front which they historically show to the world.