Before anything else, I'd like to mention that Sisterism seems to have been caught up in the overall fallout of Tsubomi ending, as this was one of their series--something which a lot of people, including myself, seem to have missed. The Tsubomi series were given a chance to wrap-up through their web-only chapters but that obviously puts a bit of a limit on the room left for development.
Anyway, as for Haruka and Yoshiko's relationship development itself, as I said earlier I thought the ending was fine. The point is, overall if you think whether Sisterism was a comedy or a drama/romance series, I think you would lean towards comedy. So if Yoshitomi-sensei only had a couple chapters left and had to decide whether to end the series on a comedic note or with big romantic drama, I think it's more natural that he would stick with the comedy.
The reason Akira/Mari, and Sakura/Mai, happened to get confirmed romantic endings was because it was more natural/easier for those couplings. But as for Haruka and Yoshiko, since there's such a big misunderstanding between them, resolving it in a quick and easy manner would be totally awkward. So if the series was going to have to end anyway, I would rather let Yoshiko and Haruka continue on the way they have, and imagine a sweeter resolution for them in the future once they've matured a little bit, compared to a forced ending right now.