Just like Civilization, Twilight Imperium and other games.

I think it would be really awesome and a different experience overall if there would be an option to turn off alliance victory.

Currently, once you are the bigger alliance, the whole game breaks down in it's challenge/reward curve. The AI is too stupid to handle it anyway, and in multiplayer, you just steam roll the rest, game over. 

Having alliances be just a tactical thing, never lasting forever would bring the excitement back into the late-game.

Basically, you can be in an alliance, but your victory progress is not counted together, but still speratedly. 

It creates nice tension, and someone has to cancel the alliance sooner or later to stop the other one from winning. 

Personally, I have very low opinion about "no shared victory" option. Reasons being it doesn't create any drama, damaging the purpose of diplomacy and making the AI worse.

 It's not creating any drama because It's expected for alliances to break down and go war with each other. It's like marrying someone with expecting to be divorced. Both parties know one of them is about to break the alliance. The when(?) question, is insignificant; 10 or 100 turns after alliance, you still expecting a "betrayal" and you should be getting ready since turn 1. As for the other, If a person thinks If he/she can be victorious faster by betraying the alliance, he/she should make the real betrayal and attack. This creates a drama. 

It is also damaging the diplomacy side. Committing a side and doing something together should be rewarded; the diplomacy in games are already very questionable due to its worth. It also makes losing separate sides to group up and try doing something together. The decision of choosing an ally is getting harder too in the shared victory. It also makes all the diplomatic options useless, why would I ever trade to someone If I'm certain he/she/it is going to use that against me I wouldn't even bother clicking the diplomacy screen in a full session. 

Also, It would just hurt the AI desition and make it very cheesy to use. You could just make an ally and next, it would send ships different place and you do "surprise" attack to It. Programming a passable AI is hard as is. 

Since I have a very low opinion about this primitive option I wouldn't put it in my game but since It's not, maybe Amplitude could put an option for it just to have more options on screen to show off purposes so you have my vote. That being said, I do think victory conditions should be increased for an alliance because you have to balance out 1v2-3-4 situations somehow but that's another discussion. 

Yeah, it is about personal preference. 

Civilization 6 goes the opposite route: They force you to abandon the alliance as default game-play. And while I didn't like it at first (due to feeling "wrong" and not the "natural" thing to do) I changed my mind after several matches, where I found out how dynamic it makes the whole game. It made the game so unpredictable and you could never know anything for sure. 

In some way it felt a lot like "Diplomacy" (yeah that age old board game, which supposedly was J.F. Kennedy's favorite board game). It's all about reading the other players mind and I know that he knows that I know he knows.... etc :)

For more relaxed and social cooperative experiences quite some players wanted to have a allied victory in civilization. I haven't checked if they actually patched it in. 

So yeah, I think it really depends how you apporach a game and your mindest about the goal of the game.

However, a lot of games I played in ES2 ended up being just a no brainer, where one big alliance steamrolled all the other players (AI or Human). A few had balanced alliances that were fun though.