If I make £3000 in interest from borrowers on a set of money and give away £5000 in interest to depositors of those same funds, even before I account for the costs of operations (systems don't develop and run themselves) I'm making a loss.
You seem to be suggestion luring people in with a high rate and then dropping it later. That only really works if there's some kind of lock-up to prevent all your depositors fleeing as soon as you lower the rates again.
If you lock-up funds, generally you have to guarantee the rates for the period of the lock-up otherwise that's a bait-and-switch, which is generally going to get you into legal problems :)
Only if the amounts being borrowed and loaned are exactly the same.
I.e 20% on $100 is less than 10% on $1000
Like a sibling comment you are not accounting for the profit that comes from the collateral.
Maybe a nondefi example would help. Imagine if a landlord got a loan using a rental property as collateral. In this example the lender will now get the payments of rent. Now the lender makes money from both the interest rate on the loan and from the renters of the property. Depending on the demand for the rental property the amount of rent you may collect can fluctuate. This means that some months you may make more money than others. So in order to sustain a certain level of profits the amount of rent you collect will need to be worth a certain amount.
[0]: https://www.investopedia.com/terms/r/rehypothecation.asp
At which stage, people with savings withdraw funds because of the lower interest rate and the Ponzi scheme collapses.
The only thing that collapses would be the APY. Everyone can still withdraw their full deposit.
The irony here is that’s supposed to be a joke and not an actual business plan.
Smart contracts doesn't save you here, since the deposit happened when you bought their crypto coins, not when you signed the smart contract.
Stealing the deposits would require
a) Verification happens without any government id (I assume this is true) b) depends somehow on the bank / smart contract authors
Then it would be trivially easy to generate huge amount of "borrowers" who simply "default" on their "loan".
Why does it need to be sustainable? The APR can simply lower once the high APR is no longer sustainable.