- Since you gain 50%, 33%, 25% or even 20% of the total amount of EXP depending on the group size
While I appreciate the effort you put into your post of valid arguments, there is still a minor mistake.
The experience gained from a killed mob is divided by the number of players the group consists of, but it is also multiplied by a modifier, which depends on the group size. You get a small amount of bonus XP for killing a mob in a group.
SourceSo, according to the example given in the source above, you gain 50%, 39%, 33% or down to 28% of the max obtainable experience, depending on the group size. While the advantages of shorter downtimes and faster kills still apply, grinding for XP as a group is still limited by respawn times and movement speed. Thus, a group of 5 probably won't kill 5 times more mobs in a set time compared to a loner.