You know what I haven't seen discussed here? What kind of land is for sale.
Based on my travels (no hard data here, sorry), it seems that land for sale is overwhelmingly General land and Moderate land in regions that are next to General regions. Following the merger of the teen grid and allowing under-18s to travel the wider world, SL residents over 18 have fled the General regions and nobody is filling them up again.
As a landlord, I have little interest in buying General land, as I have no desire to be a policeman to my tenants' activities inside their own homes.
The best thing LL could do to save the Mainland would be to bar teens from it once again. Even better, re-label most of the General regions (especially those with nobody in them) as Moderate. Then people might move back.