What I don't understand about your position on this is that you agree that if someone can't give 100% then they should leave, but (as I understand it) you don't agree that his reason is a valid reason to not be able to give 100%.
Having read the letter, that's not quite what he said though. He didn't make the link between the proposed legislation and his capability. I agree that if he is unable to do his job effectively due to his personal opinions of the changes being made, then he should go.
If he is saying "because I don't agree with the changes, I can't lead this school effectively" then he is the only person that can say that; no one else knows how he feels and whether that is true or not.
But, by leaving, he leaves the school, and the DCs, open to leadership by someone who does agree with the changes and embraces them fully. I may be wrong, but I'm not sure that is the point that the OP was trying to make. Had he remained, he could have worked within the changes. As it is, his successor could well promote the very agenda he is unable to support. That's good for the DCs if you agree with the changes, but not good for the DCs if you don't.
If, as mrz has suggested, recruitment of a new HT is difficult/impossible, then the school may be forced to consider sponsored Academisation earlier than they otherwise would do, in order to secure stable leadership. Again, whether you think that is best for the DCs depends on where you stand on the Academisation agenda.
As an aside, his assertion that he'd not even considered resignation until the Budget last month is startling. There are very few teachers, even HT, who could walk away from their career with such a short period of consideration.