Credible exit doesn't require users to actually leave.

It just requires that they credibly could.

A credible exit is effectively an ecosystem BATNA (Best Alternative To Negotiated Agreement).

It keeps pressure on the platform to be well-behaved and continue innovating, because if they don't, people could leave.

Just because it's technically possible for a customer to leave, doesn't mean it's practically possible.

That's why it's important that there's a credible exit.