This kind of scenario is exactly why Dash prioritized on-chain scaling from the start:
Your right to control your own money is absolute!
If there's a run on exchanges with Bitcoin, fees will skyrocket and users will be stuck
If it happens with Dash, everyone gets their money back.
Here's the whole post if you want to read it.
Why do people still keep imagining things like this, BTC isn't going anywhere and no such panic would ever happened to cause mass exodus from BTC.
We used to reason like that 4 years ago, and BTC keep getting stronger and better each year.
It's theoretically possible, but agreed that it's not super likely.
What is more likely, however, is that fewer and fewer people self-custody as a result of this.