I'd like to believe that even after all that happened, this was the best ending for Seki and Shimizu. Even though Shimizu lost his memories, perhaps he forgot about his trauma and can now live a normal life like he deserves, my boy even got himself a job.
Seki got over his fire ptsd, he's even smoking now lol. I like to believe he wouldn't abandon Shimizu, as he already broke his promise once and definetly regreted it because of all the shit that went down