One way they could address the trivializing of old content would be to implement rewards that are unique or meaningful even with the release of new raids/bosses.

In XI, the Chains of Promathia expansion accomplished this perfectly. It offered players a quality storyline with a reward to match at the very end, as well as zone access to Sea. The way I see it, if XIV continues down the path they currently are with the trivializing of the previous tier every time they implement a new instance or primal, they are going to end up in the same situation as WoW where all of the old content is no longer used and essentially becomes a waste of space.

I agree with the OP that old content should be built to last, so that at the very least players have the option of completing them for meaningful rewards. To those who argue that maintaining old content is repetitive and boring (by arguing that players would be forced to run them over and over again), all you have to do is continue to release new content and instances so that the game continues to grow and provides players new things to do. Giving players more options of where to raid, which bosses to fight, or which instances to run seems like the smart choice instead of releasing a vertically oriented progression system in which you throw away all of the previous content you developed.

Imagine if there was a quest line attached with Dzemel Darkhold that offered a new zone to players as a reward? I promise you that would keep the instance relevant for a long time. You would see new and old players (LS mates who want to help unlock access for their friends) continuing to run that area even if there weren't any loot upgrades.