They're both made for each other but I feel like Adachi benefits the most simply because I don't know how anyone else but Shima would have handled her. On the other hand, I could imagine Shima living her life with her other friend just fine (though not as happily as she'd be with Adachi!).
Do you mean if Shimamura dated Tarumi (the other friend you were saying) it would've been "fine"?
If you are then, I disagree. I honestly don't remember how this was told in the manga but in the original LN, the problem that lies in Shimamura is how she chooses to disconnect with other people, not even attempting to get to know her beyond what she thinks is acceptable enough to be not bothersome to deal with.
Adachi balances out that aspect of Shimamura, just like how Shimamura is unusually kind and considerate towards Adachi. It's pretty cliché but AdaShima stresses the idea that Shimamura and Adachi are two halves who complete each other. Neither of them would work with anyone else but each other.
