I love this ending so much.
Even though I saw the birthday celebration coming a page earlier, it was still heartwarming, MP100 is the journey of Mob finding himself, however the other characters are also taking their own journies alongside Mob, Reigen grew a lot with Mob, came to accept his own faults and identities, this birthday party when compared to the last birthday is so well deserved for the Master himself.
A nice surprise is Mob making the cake fly to Reigen's face, this is something i would never see past Mob doing, and with his smile we witness his amazing growth.
Thank you.<3 10/10