@Tamerlane
I agree that as far as the story goes, a harem/ambiguous ending kinda looks like the most logical one. Like you said in your previous comment, there should still be some internal conflict. I as well said in previous chapters that with them being sisters, they are bond to see Fuutaro and their love for him will be harder to forget. However, I'm not sure that a harem ending is the best one. With Fuutaro, it felt like Negi wanted to show that the Quints could act a little selfish when it comes to love. Everytime each of the five started to like Fuutaro, they distanced themselves from the Quints circle. Yotsuba was the first one to do so when she wanted to look different as shown in her origin story chapter. After her sisters followed her to another high school, she realized the bond with her sisters was important and took a more supporting character role for most of the story. Once Fuutaro made his decision she started to wonder if it was okay to have something/someone for herself. And it was the turn of her sisters to support her in their own way. So, I guess Yotsuba gain wisdom from the other parts to become waifu material. Itsuki even said that Yotsuba doesn't need them anymore. Then again, my analysis might be total BS.
@Umesan
I don't know about playing it safe, but I do feel like a harem ending is not an interesting enough plot twist. Even if the story could justify a beautiful harem ending, it's not worth it. Having a harem from the beginning to the end is uneventful imo.
Regarding the criticism, you're right. Personally I'm one of those people who thinks that the way the story was handled after Fuutaro confessed is out of character for every Quints. True Yotsuba isn't my favorite but she is a decent choice so it has nothing to do with preferences.