So we need to figure this out. Either we need businesses at the endpoints (meaning brokerages and exchanges) that are fully a part of that regulatory framework, or we need to extend the mathematical security model until those businesses have nothing left to do and can be done without.
That's simply not possible.
I share your pessimism as regards the odds of completely getting rid of web wallets, exchanges, brokerages, etc. It just won't happen.
But there is still a lot we can do in that direction to take away opportunities to steal. Businesses that are mainly doing some kind of accounting etc, don't require anything that intrinsically has to take place off the blockchain. So while the blockchain can't enforce that, eg, someone gets a pack of gum or a pizza or a suit or something physical like that when they pay bitcoin, it could enforce that, eg, someone gets a futures contract or a bond or a stock or a title deed or something.
So while the blockchain can't be extended to cover commerce in physical goods, it could be extended to cover most uses of an investment portfolio and wealth management. And in doing so it could remove well over half the opportunities to steal currently endemic to the system.