> because, when a tax starts being significant, the really rich have the means to find ways to avoid it

Taxes on property are something they cannot avoid though.

One of the reasons the rich are able to find means to avoid taxes has always been government reluctance to stop them. There are many deliberate tax breaks for the rich - think of how long it took to get rid of non-dom status, so I really do not think the government has ever tried very hard to stop avoidance by the rich.