Doubt myth will be removed. We already know myth gear is being moved to the hunt rewards. Myth will always be around for relics.
2.4 dungeons will most likely be added to expert along with 2.3's
No different than doing AK or WP with i90+ gear. The mobs don't stand a chance. It's hilarious how fast people with high allagan weapons can kill Brayflox HM trash and bosses.
I personally wouldn't want old content having level syncs. I think it's fun to be so powerful you obliterate everything.