"If we tax the rich they will just leave."

So?
Let them leave.
Pretty much anywhere they go they'll need to pay taxes.
They can't take the land, houses, resources, or infrastructure they're hoarding.
They're not doing us a favour, Trickle down is a lie.
If they're only staying as long as they can avoid contributing their fair share, why would we want them here anyway?

Either they contribute to the society that benefits them or they leave and free up their stash.

Bye bye. We can do better.