One problem with replacing Clarkson is that Clarkson is, in theory anyway, playing himself. If you watch anything May or Hammond has done outside of Top Gear you will see they also are just being themselves. And in their case that is enough.

But anyone else who is established as a Actor might not be able to bring the same level of 'realism' to the show since everyone would know they are playing a part. And if they were just being themselves would the public like what they see?

Johhny Depp is fun to watch in his roles but in real life he is sort of boring. Any replacement for Clarkson would have their on screen persona to overcome.