How about, people who actually want to live there? It sounds like a lot of the problem is "overdevelopment", that is, building second/third homes for people who only live there part-time (but whose property needs to be maintained full-time).
So one potential solution could include a tax on vacant property. I'm sure there are other ideas people have had and tried in other locations with varying degrees of success. Your statement of "I don't see any satisfactory solution" amounts to a hopeless framing of the debate that starts and ends with "we've tried nothing and it hasn't worked". The good news is that you can simply cut the second paragraph of your comment and it would be measurably improved as it would be more inviting to constructive commentary.