Aavron's revival had been successful, it was inevitable. Unless Renir or Unmeig are Feluxers, they would've no technique in stopping that revival.
Furthermore, Unmeig hoped to team up with this stronger Feluxer. But, on the other hand, he couldn't assess if Aavron being Spirit Runer is a good thing or not.
If Aavron was still stuck at being a Rune King then their collective power will decrease.
Before, they have one Spirit Runer, two Rune Kings, and one Rune Master. Now, if the two Rune Kings were not enough to make this Feluxer Spirit Runer then it was the same as one Rune Master and one Rune King were removed from their group.
But if it was enough to make him a Spirit Runer, then they would have two Spirit Runers!
It was much better than having three supports. In that way, they would have a greater chance against Renir.