While I agree that pre-rendered artwork should be done at as high a resolution as possible, focusing on DPI exclusively would be a mistake - it would result in scenery elements being rendered at the same (real) size onscreen regardless of the device (e.g. a statue appearing 5cm tall, regardless of whether the device used is a smartphone or a 30-inch monitor).
Instead, I would argue in favour of the system used by many games currently, where you specify a resolution and the game engine downsamples its images to best suit that resolution (if zoom is available, that downsampling will need to