When will California fall into the sea?

–from http://www.straightdope.com/classics/a1_157a.html

My question is this: wouldn’t the fact that continental crust is less dense than seafloor crust prevent it from subducting, thereby causing Los Angeles to collide with the Aleutian Island arc rather than “slide under?” This has happened many times in the history of the earth, e.g. in the collision of Indo-Australia with Asia. Or, in an even better example, when the Bronson Hill volcanic chain collided with what is now the northeast N. American coast, followed (relatively) closely by the Avalon terrane. There were subduction zones involved in both events.