It is not the fault of the Seto Inland Sea islands themselves that they are suffering from declining populations. It's the glossy brochures put out by local governments that are to blame.
Take the Takamatsu city brochure advertising the virtues of Ogishima. The brochure highlights a hiking course on the island, an unusually shaped rock, an opening to a hole in the ground, and some lazy flowers trying to grow anonymously — all things trying desperately to be tourist attractions but fooling no one. If you told your friends that after a hectic week at the office, you were going to hike up a steep mountain in the blazing heat and see a rock and a hole in the ground, people would think you were crazy. As intriguing as it may sound, people just don't do it.
Most people would prefer to sit in a cafe with a view of the sea or experience the charmed life and quiet solitude of the Inland Sea. They'd rather have a romantic dinner while watching the sunset or walk along a deserted beach even in the height of the tourist season. Did you know that you can do that on almost any island in the Inland Sea? But the brochures won't tell you this.