Politics is at the core of One Piece, but it's all heart instead of talking about the complicated stuff. In OP, there's this naivety of dismissing any commentary or criticism regarding models of State, governace and institutions (perhaps because it's too complicated for a work aimed at 15 yo readers?), so it doesn't matter whether it's monarchy, republic or anything else, that's not a problem. What matters in OP is whether the ruler is good or bad, i.e., whether the ruler is benevolent, selfless and good for the people instead of oppressive, entitled and bad for the people.
Why am I bringing this up? Because the Revolutionaries are only against the oppression of the WG, not against the infrastructure itself. The Revolutionaries are not against monarchy or even against the World Government itself, but only against the Celestial Dragons themselves, and only because the world nobles are rotten and produce immense damage to the world through their fascist rule. But the Revs would never aim to dethrone a good king, so that's why Dragon is against killing Cobra, since Cobra probably has a good reputation due to Alabasta being a prosperous country, etc.
The manga has spent over a thousand chapters praising these great kings, so it'd be contradictory to put the Revs against them, unless the Revs were supposed to be portrayed as villains or misguided rebels. I don't think that's the point.
