So then do we agree that the American Revolution was unjust?
Nope. Lincoln's point was about whether any self-governing community could long endure if any sub-group could drop out whenever it felt like it. The British Empire was not a self-governing community, as the British Crown and Parliament reserved the right to make laws for their colonies whenever they felt like it. Not to mention the fact that even in Britain the political system was really an oligarchy quite unrepresentative of the people as a whole.
IOW, the colonial legislatures could be abolished or over-ridden whenever the British government felt like it.
A very different kettle of fish from pre Civil War America, where even Lincoln, the evil avatar of abolitionism (according to many in the South) agreed that slavery in a state could not be touched without amending the Constitution except by that state. And of course the requirements for amending the Constitution are such that no such amendment could have passed without southern agreement.
But isn't this much the same objection of the South, which complained that the Federal government could pass laws without a state's consent? Indeed, wasn't a MAJOR factor in secession the "nullification crisis?" Again, the South remained concerned, much like the American Colonies, that a powerful foreign government could pass laws without its consent. How is that self-governing?
Moreover, the American political system was very similar to that of the British, especially prior to the adoption of the 17th Amendment. How did the revolution change things as far as that went?