The mentality of letting old content die is a bad one. They need to take advantage of the "lv sync" option for content like Extreme Trials.

They should consider the idea of adding new loot to old content that helps in new content, like say unique accessory drops in Diadem that help in Eureka. New loot in old PoTD that helps in new PoTD. rare "primal armor" drop that can only be obtained if you do it level synced.

A little nugget of a reward to entice people to try the challenge and keep old content alive.