Food for thought again, but imagine a day where ETFs allow the withdrawal of the underlying asset (BTC in our case) and many people have learnt about self-custody and try to withdraw from the ETFs, essentially diminishing the AUM (assets under management) to minimal amounts. Would they allow a massive withdrawal? We come to the same argument again. It's not our Bitcoin, unless we have its custody entirely on our own.
In such scenario, i expect they wouldn't allow massive withdraw or find way to discourage (such as high withdraw fee or scare with inaccurate information). But for reference, gold ETF is popular enough despite it's possible to convert gold ETF to physical gold (although i don't know the limitation/fee).