I mean, not every romance manga has to end with a marriage and kids epilogue. What would be the use of a marriage scene, thematically? This entiiiire manga has just been them professing their love to each other over and over, talking about their future, supporting each other, going from kissing to making love and even giving each other rings on their left ring finger. And all of their friends and family have already gathered and celebrated their love with them at different times. They're basically as good as married!! This ending is super satisfying and shows the contrast between the daily life of Moriya at the beginning of the story and now. And then this prologue shows just how much Hiyori's life was changed by Moriya as well. It's too sweet!
Can we also appreciate that from beginning to end, there was NO love triangle, despite comments crying NTR-alert at every cliffhanger. It was just tooth-aching, heart-squeezing cuteness.