As others have said, by reducing the level, all you do is increase the EXP needed per level, so it makes no difference, level is just a number after all.
If the issue is it takes too long, especially for a new player, I would think the actual wall is the MSQ. If we assume it takes 30 hours per expansion, over 5 expansions, that is already 150 hours and that is just for the Hydaelyn/Zodiark arc, not anything after plus the fact that that is likely a lower estimate.
Having some sort of secondary starting point is the most logical thing to do where I would say the start of DT is where the point should be. It is after the Hydaelyn/Zodiark arc, I don't think starting at the void is a good point, which leaves the start of DT.
I will also say, I do not agree with having an expansion 'start fresh' as, whilst it might be nice for a new player as they can learn as they go, it would feel really bad for experienced players having to start from scratch, there is already a bad stigma around low level content, so why would you force it onto everyone?
As for levelling secondary jobs, I can also see that as a detriment for someone wanting to try another job, being level 100 and wanting to try, say, Monk, which starts at level 1, that could be a deterrent. A system could be put in place that allows you to choose the starting level, level 1, 50, 60, 70, etc. with the job quests completed up to the unlock level. The caveat here is that you can only unlock up to a point, 20 levels below current expansion level cap, just like the jobs that are released in that expansion.
Really, I do believe the issue is time investment. If something takes too long, people will likely be put off by it and masking it behind something else (halving levels) doesn't address the core issue of time taken. By showing you have ways to skip parts of the time taken, it might draw more people in, especially if it is also made clear that there is a system in the game where you can replay the bits you skipped if you want to.
