Why not require people to acquire mats from older instances in order to obtain the endgame gears? Instead of awarding a piece of gear straight up, make it so that people only get a unique material (from alex nm and alex savage) and then have to obtain other common mats from older fights like levi ex or turn 8, then craft or exchange all the required mats for said gear. This will keep older contents relevant for as long as the game is around.

Ponies and glamours are not enough incentives for the masses to go back into these older instances. Five years from now, if an i1000 piece of gear requires mats from titan ex, people will still have to do titan ex in order to obtain it. The game will feel a lot "livelier" since there's always a lot for everyone to do. It will be sort of a grind just like how we have to grind 2 dungeons for eso every week right now, but at least we get to do something other than just running two dungeons.

Of course, all fights should be level synced to prevent people from one shotting the mobs and provide a challenge.