@lvxferre When the whole story is about a glorified horse race analogy I think it's reasonable to be angry if the horse you were betting on lost.
Compare it with other romcoms like Hi Score Girl and the MGT one which have the nostalgia as selling point or Kaguya and Jitsu wa, which made a point of showing teenagers having actual friends and interactions, or School Rumble and Seto no Hanayome having y'know, actual comedy. Ask yourself this. Does 5toubun deserve the comedy tag? Did it have anything of note besides the quints? Was the bride mystery, the main selling point of the manga, satisfying?
That last point is important because for the people who wanted their quint to win, the main selling point of this manga was a disappointment. Who won, unlike other romcoms is a bigger issue here because the manga failed to give more reasons to enjoy the manga. What it did, it did good, however that placed a bigger strain on the ending than a normal manga with more charm points would have had.