To be honest, they sort of subtly pointed towards this direction with the whole 'we're going to end Reversi as a masterpiece' thing. I mean, they already said that it was pointless to drag on Reversi after the fight between Schwarz and Weiss, and the main driving point of the manga was Mashiro and Azuki's long love story.
But I agree that the ending was not the most satisfactory. It had symbolism, with the house and all, and it came a full circle, but the ending was somewhat open. Who knows? couples break up all the time, even the closest of them. I think showing their wedding or their life after the wedding would be a great kind of closure.