I don't think bitcoin can be 51% attacked. However, it could be rendered unusable due to etchings on sats. Who would want to do this? If a state actor wanted to stop bitcoin from challenging the USD, they could simply print to infinity and buy rune block space pushing $100 dollar UTXO's with $10,000+ transaction fees. BlackRock could certainly manage this and they would be getting their own money back since they own the miners as well. The question is, how long could they do this before the USD is toast? Would love to see a math formula for that.
edit:
Or maybe Blackrock eventually ends up with a lot of the bitcoin if they pursue this avenue (in the form of fees). They were too late to corner the market. But what happens if the value moves elsewhere to where there's less friction? Then BlackRock has sent itself into oblivion.
The biggest flaw with this angle is the assumption that Bitcoin challenges USD.
How is that possible?
The ultimate function of a fiat system is that debt is easy to create and that debt has a stablish value. Bitcoin, and in fact all crypto, is not trying to compete with these functions. In fact Bitcoin is the most pristine collateral the world has ever seen, which is something that all debt-based economies have been in desperate need of. So not only does Bitcoin not compete with fiat; it helps fiat quite a bit. This will become more obvious as the bankers start creating Bitcoin derivatives... the first of which has already been created in the form of an ETF contract.
Just because the creators of crypto said it competed with fiat doesn't mean it actually does. In the end it makes sense that permissionless systems that bankers could use for their own ends was actually going to help fiat and debt-based ecosystems; perhaps more than any other technology ever invented.
At this point I guess even if Blackrock would own 10% of all BTC, this influence would not be enough, but they are far from that.