If they do significantly nerf fate xp, then they need to boost the xp you get from dungeons.

Also, as others have said, if a group of 8 people chain grinding is better xp than fates, then suppose we take the say 100 people that chase fates in Northern Thanalan, divide them into about 12 groups of 8, and give each group a chunk of zone space where a sufficient number of mobs exists, and respawns at a fast enough rate to make chain grinding worthwhile... we would run out of room.
The zones cannot support 100-200 people, in groups of 8, farming regular mobs, the mob density is too low, as is the respawn rate. So groups would spread out over larger areas, and eventually start overlapping and fighting for spawns... at that point fates are better again.

So, that leaves dungeons.