The value of an American citizenship is incredibly high. The vast majority of people in the world would go to great lengths and expense to be able to have one. The taxation requirements, as onerous as they are, are still worth it to the vast majority of citizens.
Similarly, the value of a British citizenship (or of any other powerful western country) is very high, and most people would put up with great costs to have it. The author mentions that he pays quite high taxes there as well.
The marginal value of having an additional citizenship from the set of powerful western countries is quite low, though, which is why this author no longer feels that the costs are worth it. But only an incredibly small fraction of people are in a position to be citizens of more than one powerful western country, which means that the costs of being a citizen will likely continue to be judged (by most people) against the value of citizenship on its own, not as a marginal addition to some other citizenship.