I think that most of forget that Mifumi isn't in control of the series as a whole.
They can't do things like stating (moto) Takagi's and Nishikata's first names, they definitely can't do anything that would lock down the plot for the main series, and they can't show much skinship between the two.
The last one is pretty important since, like people have said, that would break the tone of the main series.
Given that they aren't allowed much freedom, they've done a pretty good job at writing a relationship between (moto) Takagi and Nishikata.
There were hiccups to begin with, early Nishikata was basically a bigger version of his middle-school self.
Lately he's been at least slightly mature, it's easier to pass off his reluctance to be close with his wife as "honey, not in front of the kid" now.