I agree mostly with that most most of Yotsuba's development is condensed into one or two chapters as opposed to the amount the others got, which I think under sells it a bit. I think if Negi really wanted this ending, she should have paced it out a bit better and really fleshed it out more, as this really feels rushed overall.
Whilst a harem or ambiguous ending would have been predictable, sometimes just because something is predictable, it doesn't mean that it worsens the story for it. In fact, sometimes I think that things like Man vs Fate as a conflict or the fact people know what is going to come and the author plays off of that is the best thing for a story. It's cathartic for readers to experience when Odysseus is able to return to Penelope and slay the suitors after all the shit he went through, for example, even though it's evident that it's most likely going to happen.
Negi's biggest fault is that he didn't construct the narrative to fit this type of ending and didn't focus in on how it, in the narrative, was going to function. It ironically feels more like a cop-out ending now in my opinion than the more predictable ones
@Kefkabot