Let me tell you how this plays out, from an Irish perspective.
1. G7 countries can't do anything, trade treaties are sealed, Irish taxation rates are their sovereign right. Vetos baked into things because of the first rejection of the Nice treaty.
2. Even in the event of international pressure forming, (And the US can of course strong arm an arm or two), I don't think the international state understand or wish to see where Ireland is coming from. A country with no population pool, geographically isolate, and no real natural resources to exploit and a worker population very demonstrably willing to flee at the first sign of a failing economy. Enforce this rate, the only real thing that is seen to have made Ireland not a back water farming country, and Ireland engages in what it's government has ever really achieved successfully. Invent tax loopholes. The second an Irish government agrees to this, those politicians will be metaphorically taken out back and shot in the head, to be replaced with someone who won't doom the economy. Even the former terrorist party if needs be. And then they get straight back to the economic gurella warfare, because there's no chance of taking on the G7 mono-el-mono. Every system can be hacked, every tax law has a tax loophole. Ireland just needs to look hard enough, if incentivized to do so. And it's entire stability and economic future is quite the incentive.
And if they do, it will be a shattering of taxation sovereignty and a fundamental dividing line on what makes the EU a group of cooperating states instead of a unified single country. Hungary and Greece alone would leave the EU over that.
I think you are misinformed about the level of corp tax in most poor countries. Twenty countries have corp tax less than 15%, of which about half are poor. 15 have no corporate tax and are either rich or are just unambiguous tax havens.
Average corp tax in Africa and South America is 28%, Europe and Asia are 20%.
Most of these are poorer than G7 (and especially the US who is the main beneficiary), so my statement is 100% correct. I didn't say that all poorer countries will get poorer by this specific action. There are many other actions to make everyone poorer than the US.
And less private spending and investment by these corporations and their shareholders. The optimal size of government spending, as a percentage of GDP, for economic growth, is likely well below 25%, and significantly lower than the level in every OECD country, so to the extent that this enables increases in government spending, it will likely hurt economic growth:
