Alistar is a staple of this game. That's why he's in every worlds.
It's insane that people assume a champion is a problem just because the champ continues to be picked. Do you think once a champion sees pro play it should be gutted so next time we can see a different set of champions? It's good to have competitive staples. It provides stability. If you look at any other competitive game that stands the test of time, you'll see that balance patches aren't even in the equation because most of them are older games. And the top tiers don't shift, but it's actually good because there's metagame stability. Instead of having to relearn the game every year players can actually hone their skills to an insane degree.
I'm fine with LoL having balance patches but it needs some stability. It's good to have some staple picks that are solidly useful and see play every Worlds, especially when they have straightforward kits and straightforward roles.
So don't assume just because pros continue to play champion that they think it's broken. Maybe they LIKE that champion. Insane concept, I know.
PS: As for the Thresh comparison, you'd pick him if you want to have an insanely high utility champion with limitless potential. The catch is he requires the player to get closer to the limit of his high skill cap to really stand out. Alistar doesn't require as much mastery but that's fine because some players are more strategic. The game doesn't require everyone to focus really hard on mechanical skill and that's also a good thing. I think if you complain about that you're just too lazy to put in the work to master a champion like Thresh.