In your example, it is like everyone who has EUR being entitled to 1:1 notes of a newly-created currency called lira, with owners simply entering their EUR note

In your example, it is like everyone who has EUR being entitled to 1:1 notes of a newly-created currency called lira, with owners simply entering their EUR notes' serial numbers and being credited the lira. Your "forex brokerage" then decides they're "not going to mess with this newfangled currency", leaving you none the wiser with how they're handling the newly created funds if at all. If it goes up, they could shrug and say that they had the serial numbers so it was theirs the whole time. If it crashes, they could credit everyone the decreased value of the lira out of money they made by selling off earlier and pocket the rest. (Or take option A and pocket all of it, if the 'good' PR is not worth the slightly decreased profits.) Considering the prices people end up having gotten "locked-in" on after price fluctuations when using Coinbase, this wouldn't even be "far-fetched" for them so much as par for the course.