Having just traveled through a range of cities and towns in Japan, I don't think the lack of options for shopping and restaurants are the reason that small (150,000+ population) towns are depopulating.

There's plenty of ways to spend money in them. That's not the problem. Neither is there a huge lack of conveniences. Any one of them is way more livable than almost anywhere in North America.

Poor economic prospects for the area is likely a part of the problem. The big cities pay significantly more for ~the same work.