I don't think you understand bitcoin as well as you think you do. 51% attacks do not work like this.
So yes, 51% control could be usurped by a dedicated attacker - but the resources are no longer trivial. And as noted, this "control" doesn't buy you a great deal unless you keep it going for eternity, and for the time period you do have control all you can do is basically stop it working properly by preventing new transactions - rewriting history is exponentially more expensive the farther back you try to go.
For sure these banks wouldn't use their own hardware, but hire a new team, new hardware etc.
If this were a big transaction, I might choose to wait for more than one confirmation before completing my end of the transaction. The longer I wait, the more expensive it becomes for you to conduct your 51% attack.
It really is prohibitively expensive, to conduct such an attack, when you consider a few other factors. I could double spend the money to myself, but after news of such an attack came out, the price likely could crash, reducing the prize.
Not to mention opportunity cost. If you have the means to conduct a 51% attack it means you have a lot of capital and expertise. You could put those two together to make a giant pile of money through legitimate means, like operating a hedge fund. The risk is a lot less.
The only group that would be motivated enough and capable enough to perpetrate such an attack would be a state, and why would they bother when, like was mentioned elsewhere, they could attack bitcoin with the stroke of a pen, via regulation etc. It's waaay less effort.
Also, would like to note that Bitcoin can also function as store of value just like Gold.
Any news about Lightning? Is that running live now?
Completely wrong. There is a set of rules which define whether a Bitcoin block is valid or not, things like "the block's hash has this many zero bits" (simplifying a bit here, the actual rule is a bit more complex), "all transactions have a valid signature" (simplifying a lot here, it actually runs a sort of a small program) and "no transaction spends outputs which have already been spent". It doesn't matter how much Bitcoin you have or how much hash power you have, even if you have 100% of all Bitcoin and 100% of all hash power, if the block doesn't follow the rules it will be ignored by all validating Bitcoin nodes.
What having 51% or more of the Bitcoin hash power gains you is only that you can rewrite history. You can present a chain of blocks to other nodes, and later present a different "longer" chain of blocks to the same nodes, and they'll accept the "longer" chain and discard the older one. All these blocks, however, still have to be valid to be accepted.