They actually haven't said how many alliances you can have. So this could mean multiple. Even if it is only one, this will effect the game. It will only take 4 tribes then to slotcap a server amd force other smaller tribes to joinsim. You are still creating the same scenarios that happen on regular official. Or if the tribe having to defend only has 10 members on that is a 5v1 fight, where as against 25 there is still a chance of defending.
Anyone who thinks that official isnt in a decline doesn't really play. People are trying to get away from mega tribe/alliance life. They are bored. This was going to be a fresh start. 25 people trying to make it through ark. Not having to worry about going to help defend / or attack with another tribe because of alliances.
Having an ally tribe also give people the opportunity to get around the bodybag rule. People will make an ally tribe with their alts. Inactive people are still in the tribe. Have fun enforcing that.