It makes sense, but one thing that concerns me is Imu just obliterating islands now, completely, which seems to be the end goal of the crazy mf's plan. So it can't be solely for resources, unless Oda Imu is really stupid and doesn't realize the planet is becoming more flooded with every use of the Mother Flame.
Anyway, the celestial dragons go after nations not affiliated with the World Government every three years, and the WG has been active for 800 years, that means they've likely destroyed around 267 islands/nations right? If the OP world really does have 20 million islands, each with their own nation rather than multiple islands under individual nations, then I guess most of the world won't notice if an obscure island goes missing.