Technically you can continue introducing new content as well as class abilities without requiring leveling, but it can restrict new player access to content severely in the long-run.

Say a fairly new player is looking in to an end-game linkshell 2 years down the road. Would that LS be more likely to take a player with mediocre gear from 1 or 2 tiers ago, or a new player that they have to drag through 8 tiers of content just to get up to speed?