It's... impossible. It is in my opinion impossible to go directly from Bitcoin to... more Bitcoin.

99.9% of people still use fiat on a daily basis, maybe in 20 years it will change but we are nowhere near that point.

A 'parallel economy' implies that all things everyday now and tomorrow are priced in Bitcoin which is not true. They are priced in dollars, then euro, then yen probably in that order...

And as Bitcoin outperforms the dollar (which it has done for 15 years) more people will learn about it, save in it, ultimately custody it, and eventually spend it.

But the process to 'replace banks' will take generations and more likely it's like Bitcoin 'eating the banks' as they themselves use Bitcoin more and more.

A parallel system separate from banks... is impossible because it's impossible to separately price goods and services from the broader market.

Which is first and foremost looking for stores of value.