Mate that isn't how it work. If Coinbase or Fidelity don't recognise the real blockchain then they no longer have real bitcoin, they have a forked coin (because they are not on the true bitcoin chain). The Bitcoin chain is determined by consensus of the majority of miners/nodes (not bitcoin holders!) and if they break from that they now have a substantially devalued asset (look up the price of BCH and BTG these days).
You seem to be under a misapprehension that owning the most bitcoin gives an institution or individual power over the network. It does not.
Bitcoin, by definition, cannot exist on two separate chains. If an institution attempted what you are saying all they will have done is reverse alchemy: turned gold (bitcoin) into lead (an unrecognised chain with no mining occurring, no recognition by nodes etc).
again, the cheerleaders who might know math (usually not even that) forget about the ruthless of business
https://www.investopedia.com/terms/1/51-attack.asp
> forget about the ruthless of business
Noone has forgot a thing just aren't clowns like you who think businesses are some substitute god who can't be beat. Fidelity doesn't "hold" the transactions. Transactions to be relevant MUST be broadcast to the network and if this doesn't happen then the blockchain plods along as if they didn't happen.
Honestly, learn a bit more about how this stuff actually works before commenting.