Space needs to be both less gamey and rawer. Closed border is something you can enforce in an organised, UN like environment.

Who's gonna stop my science fleet when I have 3x power than you? Who's gonna enforce that closed border?

That's just a gamey shortcut to implement some way to limit you. It's a preposterous hard limit. If you don't want the player to ever become overwhelmingly powerful, change your gameplay.

Also, it's fucking space, it's empty, it's wild. We're in the prehistoric era of space civilisation, there's no thing as a border, there's no Metternich in space. When we look at the scenario Stellaris provides, we must look at our Bronze Age.