That control may not be visible by the time the operation completes.

Toasts are popular, but not the only option if you want to notify the user about completion of a longer-running action when the user may have already switched away from where they started it. Consider a status bar[0] instead. You can make it cute and animated, too!

[0] https://developer.mozilla.org/en-US/docs/Web/Accessibility/A....

There are certainly times where this must be the case, but I think it's broadly better to have designs avoid this.