they're all equally friends. if Nirei can have it worse, why can't Sakura have it more? in case you forgot, he's the MC for fuck sake so all of the resolution should be coming from him that's how a story progresses. it revolves around the MC and not from the other half of your ship smh.
If you go on and about complaining why then write your own story. don't let the creator question their own story progression. it revolves around their idea not yours. at least be thankful they idealized it.

go read Kotoha's words again and again. HE HAD NEVER feel loss before since he was alone all the time. he had no one to rely on and when he did and opened up to people, he was suddenly being left behind by someone whom he could count as a person he can connect with.
do we all need to FURTHER explain why?
Nirei should have had it worse? no. Nirei is mentally strong. he is stable. while Sakura isn't. even from the beginning, Sakura's mental state isn't strong. and it was hit hard by Suo's departure.
saying that it should be focused on Nirei since he's closer to Suo? hello? where? when? did Suo let it happen? just because he taught him defense doesn't mean he considers it as close bond. Suo honored their master-disciple relationship when he show up in front of Nirei's face and land a punch.