Regarding fungibility attack I think it’s a valid concern, however keep in mind that they will be limited to geographical zones and it's laws (USA & friends) in this case.

There is the possibility to move to another sovereign nation where there is no concept of “tainted" sats in it's law. This was not the case historically with gold or fiat for important amounts.

Blackrock trillions of paper money is not that important as it seems, they at most can become a private club holding 1 millon BTC.

The big game will be a global one, where countries see the geopolitical importance of Bitcoin and compete to mine and accumulate reserves of it. I suspect this is already happening at some scale since mining pools are very well balanced between USA vs Rusia/China and hashrate keeps going up.

So to effectively kill bitcoin you will have to form a "world government” and censore the tx's controlling the whole network. Even in this dystopian scenario alternative out-of-system markets will emerge as they always have.