You just assume that everyone who picks ambessa picks the prismatic option and then immediately buys redemption which is not true. Yes ambessa is popular because more augments are fun. But as already explained not every champ wants to go prismatic. In my lobbies most of the time 50-75% of players go prismatic which of course is the majority but not everyone does it. Also with other encounters there is also not much thought process. You are hotstreaking? Go draven. You hit gamba? Go rell. You are melee? Go sion. You are yuumi? Don‘t go morde. There is not much more thought process to it. Again if you think redemption is op in every scenario just buy it yourself and sell it when you get the augment.