Why? If a country can be more efficient, why must they be penalized by being required to raise taxes? This is the equivalent of a price floor. Why should a country be required to have higher taxes to appease those that make different policy decisions? It should be up to the government (voters) what tax rates work for their country. Why wouldn’t a tax rate ceiling be proposed instead? Why should Ireland raise taxes just because France wants to run huge healthcare deficits or offer extremely generous train worker pensions? Seems like decisions on tax rates should be left to the country. If a country wants a 50% tax rate, that’s their business. If they are economically harmed by someone else having a 10% rate, then the problem isn’t that someone else has a lower cost but that they have too high a cost.
As far as the 2/3 rule for US treaties, that’s a good thing. That ensures that treaties are good for the entire country rather than just a simple majority. I don’t want 51% being able to ignore 49%. If Republicans were in power and proposed a maximum tax rate treaty, those complaining about the 2/3 rule would be singing a different tune. The Constitution was designed specifically to ensure that a narrow majority isn’t able to run roughshod over everyone else. Gridlock is a feature, not a bug. And that feature benefits everyone at different times.