New worlds are introduced every month as old worlds fill up. As you have noted, older worlds are dying or are already as good as dead, in the main. They have not been forgotten. It seems quite regularly it is suggested to the developers ways of enlivening old worlds or making new ones more challenging and interesting. I think some of the ideas posted will eventually be adopted, but not quickly as it takes time to evaluate the effect the changes will have on 'new' worlds as well as old ones.