Only the node runners can cause a fork. And they won't. Because they're not cucks.
Discussion
Technically they have so much money that they could put pressure on developers and miners, BlackRock is BlackRock, BlackRock decides the direction of many companies and even nations, don't underestimate them.
And how much bitcoin do they own? There is money and there is money.
One source states "Around 8% of all the Bitcoin in the world is owned by governments and companies."
The hardcore, anti-establishment bitcoin army is not going to give up their BTC easily and there is little bitcoin left to scoop up (whatever is being traded on CEXes + what's left to be mined).
BlackRock by paving the way to institutional adoption and pumping the price will shift the balance of power towards the aforementioned bitcoiners and away from itself.
I think that if they tried to drive the fork solo, it wouldn't matter how much they have. It would just become BCBD, and be much like BSV. Nodes and amount of BTC are really just proxies, like hash rate was. I think the real chain is the one accepted be the people selling the most value. They would be holding the bitcoin backing other people's paper bitcoin, and they are BlackRock, so there might be an attack vector there, if they front loaded it with a massive drive to onboard no coin's to custodial bitcoin, and then controlled the licensed custodians? I'm not sure, but it seems like a viable attack vector.