People like this who advocate for open borders would probably be sorely displeased with the results, and those former supporters like him with the financial resources would quickly migrate to countries who didn’t self-destroy with such policies.

As a commenter noted:

There may be other examples, but they must be few and unusual, precisely because no state that is strong enough to control its borders has ever permitted the kind of immigration experiment you propose.

Anyway, talk of “open borders” is just signalling — it’s to demonstrate your open-mindedness, supposed empathy and cultural sensitivity. As noted in the comment above, no nation with the capability to control its own borders would allow such a thing to occur because of the absolute chaos it would cause.

In other words, “open borders” is supported by people on whom it’d have little to no impact either through their own mobility due to affluence or because supporting it currently has no cost, only benefit for signalling purposes.”Open borders” is one of those things that people of the right mindset (that is, neoliberals who believe they are progressives) like until they are forced to try it.