Lets look at why people still do ARR hunts.

Allied Seals: My own personal reason is this. On my alt, I still don't have my caster outfit associated with my GC, so hunts are really the best (maybe even the only) way to get them (other than leves).

Achievements: This speaks for itself really but other than people really wanting full completion, I don't see this being a factor to the problem.

Reward items: This is mostly for world bosses I assume. I don't think a lot of the S Ranks drop anything of value but this might be a little bit of a factor too.

The Mount: I suspect this is the real reason for all the ARR hunt shenanigans. If I'm not mistaken, the Clan mount is based on completion of hunts of a certain rank, no matter the level. If this is the case, capping this off at lvl 70 hunts only would likely stop interest in the ARR ones, leaving those who want the curreny and achievements left to put forth the effort.

Just my two cents. If they had a currency exchange for Allied Seals (like from clan seals) I'd never bother with an ARR hunt again.