@reu That doesn't really address my point that "Senpai" is little more than a cipher. I'm all for a well-done hetero pairing, I'm less enthused when one of the pairing has about the same amount of depth and personality as a square of drywall.
To use an analogy you might understand, forget about the genders of the participants for a sec and assume it's a guy in an erotic visual novel having to choose between two routes: one is a character with a complex personality, motives, history etc.. The other is just the crusty sock he sometimes makes use of. I don't think you can seriously argue for the "sock route" being the preferable one. Even if the main character waxes poetic about the sock and how it's always been there for him, it doesn't mean the sock is any more likely to stimulate you the player/reader. Same goes for this senpai character -- whatever the main character feels about him, it doesn't transfer to me the reader at all. The writer should ideally be making me feel the same way about this guy. If those feelings are as obvious to experience as you say, that means the writer had even less excuse for dropping the ball on this.
Maybe it'd bother me less if the wife was less nuanced/characterized as well, but that's not the case. For the record, I'd be saying the same thing if the issue were flipped. If the wife was a paper-thin pastiche and the senpai was a developed character I'd be going "drop that battle-axe ASAP this is your fresh start with this awesome guy! it's so frustrating the story keeps going back to her!".