So you expect people just borrowing you coins worth of millions if you promise to pay back?
With a legally binding and notarized contract, of course.
This funny thing happened to me two years ago.
I wanted to buy something non-virtual. It was pretty expensive. It has four walls and a roof, and all sorts of nice things like heating and cooling and electricity. It's often referred to as a "house."
I didn't have enough cash to buy it. So I went to some people and I asked to
borrow hundreds of thousands of fiat dollars. I offered them a 3.5% annual rate of return -- something comparable to what I discussed above -- and proved to them that I was a pretty good risk and was likely to pay back anything I borrowed.
And you know, the funniest thing happened: they said yes.
Who'd have thought?
Hundreds of thousands of fiat dollars is not very close to 51% of all dollars that exist, or am I mistaken? And at least where I live, banks don't give the money to you, they transfer it to the seller's account in exchange for a document that the bank will hold which basically means that the bank owns the apartment/house until the loan has been paid back (sorry I don't know the English terms but you get the idea). This guarantees that the money has been spent on the house, and that the value of the house "backs up" the loan.
This is why I provided the example of Soros and the bank of England. If you adjust for inflation and the GBP/USD exchange rate at the time, he borrowed substantially more than the entire current market capitalization of Bitcoin (which is only about $8b USD now). Soros borrowed the 2014 equivalent of about $20b USD.
It's very easy to borrow money if you already have a lot of it, particularly for short terms. And remember, in this case, you don't even need to borrow the money - you're just borrowing one of its capabilities (that of staking and verifying transactions). And, as AlexGR pointed out, many of the coins in the network are not actively staking.
If the profit margin was 50% and the volume was right, it's naive to believe nobody could borrow a billion or ten to exploit it. They would.
But if that's a possibility it's easily prevented by people knowing that it's possible and not lending out 51% of the coins to a single entity.