I'm not sure I follow this argument. Flightless birds happen on small island when those habitats are free of significant predators, for example after some extinction event or due to volcanic origin. Birds can still reach the islands, at least for some geological period, and evolutionary pressure favors some of them to grow in mass, bone density etc. to better control the available land food sources such as seeds, grasses, insects and other small critters.
Flight is not a useful trait in this environment and is expensive to maintain; a flightless bird will always win a fight against a flying bird for the same land source of food, since flight requires elongated and brittle bones, a low mass of muscles optimized for endurance etc. So flight is selected out of the land feeding niche very much for "efficiency" reasons.