all you're doing is letting the bank (and the borrowers) choose the uses of your money for the first while,

You're letting the bank and borrowers choose uses which they expect to be worth more than the cost, under the knowledge that they may be bankrupted if they choose poorly and keep the surplus profits if they choose well. These constraints tend to lead to fewer consumable luxury purchases and more carefully selected productive investments, and having more of the latter increases the potential economic output of the future.

There are many caveats to this, though. Does our potential economic output really have no upper bound within a hundred orders of magnitude of its present state? That seems unlikely, but if not then those exponential returns are just the bottom tails of S-curves. Is this economic system going to be protected from overwhelming corruption, violence, and theft for a future period longer than all prior human history? That would be historically unprecedented, but it only takes one disaster to wipe out a fortune.