geto and gojo went on that mission together and saved miminana then they graduate and become teachers, toji is working through his gambling addiction and trying to raise megumi and tsumiki with shiu’s help, haibara barely missed a fatal blow on that mission and he and nanami brought the souvenir sweets back, riko and kuroi live in okinawa and are safe, and NOTHING BAD HAS EVER HAPPENED TO THEM.
riko amanai's death also being the literal and proverbial death of satoru and suguru's innocence. there's no need to explain this but god. like. there goes their promise to keep her safe if she chooses to not merge. there goes their youth, that brief stop they spent in okinawa. their purpose, to save non-sorcerers, their moral beliefs, gone with riko. there goes their assurance, the camaraderie that they know they're the strongest. because how can they be the strongest when they cannot preserve one child's wish to live on? who's only here because she's somehow pushed to have a pivotal role in preserving society's balance, just like them? what's the point in preserving society if they cannot even save one.
"strongest" is a farce when they're not strong enough to keep their promise, to keep each other safe, alive, and a girl who just wants to life a normal life, all of them robbed of their youth.