This is a G7 agreement, so it seems the success of the policy will depend how much sway this will have with G20 countries. If it doesn't get the G20's agreement

This is a G7 agreement, so it seems the success of the policy will depend how much sway this will have with G20 countries. If it doesn't get the G20's agreement, what stops multinationals from simply domiciling themselves in another country like South Korea or Australia? It's currently done in Ireland, so this isn't a big jump. Also, there are countries that are not in the G20 that could quite easily undercut an agreement, notably, Switzerland. It already acts as a tax haven for personal income (and corporate income), so an expansion of this haven seems forseeable. Despite this, I don't think this is a reason to not act. There will be reasons why multinationals will not want to be domiciled in Switzerland.