I would like to see an argument for that. With the original scenario, its just an argument for autonomy and that less homogoneity and centralization in governmental structures better represents and distributes power amoung the diversity of the governed. Someone arguing for Brexit and against Calexit would have to do so on other grounds - which I'd be interested to see if they have any weight.

The things that make a region a part of a country are different from the things that make countries a part of a transnational union.

Seems simple to justify: Brexit is just canceling a treaty, the other is dividing a country.