With end game content people spend typically far less hours getting to the top level and then play together every day at that level. They run different dungeons together, play alts that they can get to max level and then typically have six months or so to play together befor and expansion comes out. You spend that six months perfecting your end game content raids/dungeon teams and getting better gear and hopefully have it mastered when the expansion comes out and are ready to take that on. Then when it does come out they are at the same level and can do it together.
As much as I like questing and leveling, the game is too young for level cap increases. I would have released Craglorn without the level cap increase, that way VR10 could make trials/raid for better gear and consider that activity high end.