The writing was not the problem.Does the biggest problem lie in the decision's lack of clarity for which could lead to which ending, or the fact that the decision itself was kind of insignificant in terms of the overall plot? Or maybe the writing in the bad end was just shit, I dunno. XD
Regarding significance, I don't really think decicion points are at all suited to the medium of fanfictions - I've extensively explained why I think that in other places - so if an author insists on including one and it doesn't even turn out to be significant I find that mildly frustrating.
But yes, the worst part was that it was so random. The resaon most authors give for including choices is that they want to emulate the feeling of a "game" that a VN supposedly is, but a game is supposed to reward the player for "good" choices and punish "bad" choices. It can even include insignificant choices - if they don't punish them. Character death is in fact the ultimate punishment in a game, and to have that after a choice with an unknowable outcome is just bad game design.
As brythain said, it depends on the genre/setting.Also, just to touch on suspension of disbelief, I think the reason I have an issue with that is because my suspension of disbelief can be stretched incredibly far. I generally won't question something in stories, video games, or movies if it's interesting and/or entertaining as a whole. It's probably for this reason that I tend to lean into more fantastical territory when writing stories, since in my mind it doesn't matter too much that it's not believable.
In fantasy settings I'm inclined to give a pass to a lot more than in hard scifi settings.
In comedy settings I won't criticise roadrunner cartoons for their depictions of physics.
But this story was a serious romance/drama with no supernatural elements up to this point and as such I apply higher standards than that.
So If Hisao is standing on the other side of the road and a sports car is barreling towards Emi at top speed without even having time to engage the brakes, I WILL question how Hisao manages to cross the entire street to push her away, etc.
The driver not even stopping and the biker not noticing the whole event just excerbates that. It would have been a bit more believable if Hisao had managed to push Emi to safety but was hit by the car himself. Then you wouldn't have had to include the biker as an additional unlikely element.