Turns out we need performance on the front-end, and ease-of-development on the backend, not the other way 'round.
Really, though, the strong motivating consideration is that while backend code scales horizontally (i.e. you can throw money at an app and get more concurrency), the user only has one computer, so you have to optimize frontend code for performance. If we could just pay money to make users' computers faster I guarantee we wouldn't be worrying about browser JITs. :)
I mean, I understand the reasons and more than anything I'm excited. But it's still one of the things that if you time traveled to the 90's and told some developers about the future, they'd lock you up in a mental asylum.
Really, though, the strong motivating consideration is that while backend code scales horizontally (i.e. you can throw money at an app and get more concurrency), the user only has one computer, so you have to optimize frontend code for performance. If we could just pay money to make users' computers faster I guarantee we wouldn't be worrying about browser JITs. :)