I understand your concern, it is a valid one. Sadly I think you are on to something with SE and how they seem to favor making throw away content, you do it when it is relevant or not at all.
There are no rewards and therefor no reason for long time players to do said content synced, this is something I find to be rather sad seeing as there is so much content that due to this is left as "dead content". Dungeons, trials and raids are all left to die once they are no longer relevant, for dungeons this is usually when they are out of the expert roulette, for trials it is the next patch or when most people have the mount, for raids it is the second the next tire hit... sad but true.

One way to perhaps help with this problem would be through a "help needed in old content" sort of thing, you get to chose if you want to help and in what content, on completion those helping could get a sort of currency that could be exchanged for glam, emotes, pets, mounts, or housing items.