That's not how things actually work. If it was, every nation but one would lose all of their corporations to that one country with the lowest rate.

The decline in the benefit of a lower corporate tax rate accelerates as you go lower. For example, going from 35% to 25% might reduce your corporate inversions by 75%. Going from 35% to 10% might reduce your inversions by 85%.

The challenge is to find the level where you gain the most as a nation overall. If you have something to offer other than just the low tax rate, it will never make sense to chase toward zero. The closer you get to that ideal level, the more other factors come into play to your nation's benefit in regards to keeping companies. The US has vast benefits for a company like Pfizer that would compensate for N% additional tax rate.