My only concern with this is that when new lvl caps are introduced they often make adjustments to lower lvl skills and traits, and the jobs are usually balanced around their full potential at lvl cap. If you have hard contents below lvl cap that can't be unsynced, then the devs need to take into account balance at lvls below cap, and that's extra work.

For example right now they'd have to balance lv 60 synced jobs against the old 3.x counterparts if we have unsyncable lv 60 hard contents, otherwise there's a possibility that those contents are too easy or too hard to clear now with our "new" lv 60 jobs. Even back in 3.5 if you go min ilv gordias it won't feel the same as back in 3.0-3.1 due to job adjustments (pld dps buffs, ast card and heal buffs, mch and brd buffs from 3.2 and 3.3). Ofc you can argue that 4.0 was an exception since it changed so many stuffs like cross class system, job gauges, major changes to jobs like brd, mch, war, while 3.0 changed much less when it comes to lv 50 jobs/contents, but I'm certain we'll see expansions that change many things again in the future, even if it's not every single expansion down the line.

Is it worth the extra effort for the devs to keep rebalancing old contents for new jobs, and/or rebalancing new jobs for old contents?