While I agree that a lot of players won't run old stuff as much, and that many players will just run the fastest/least annoying dungeon to get their tomes (I see this every time the week resets and I spam roulette and it's fractal every time for the most part). I wouldn't mind having old dungeons be level synched up to your level it would give the game more options. I know I would run those dungeons even if I had to wait longer than necessary

Off the top of my head I would run Tam Tara HM, Lost City, Pharos Sirius, Qarn HM, Aery, Vault.

Since when having more options hurt anyone? You can still run the same stuff you want to run, but let others run stuff that they enjoy being in. It would be an easy fix too just scale the hp and damage of the mobs and bosses.