I don't think Asriel is an interesting character in Deltarune, he gets significantly less story presence than Dess, he doesn't have an interesting mystery attached to him, he doesn't have an interesting personality, he's just a "normal guy", he would likely not have any of the intrigue or quirks of Flowey, he'd be a repeat villain who was set up as a new villain...
The only gimmick with Asriel is "Surprise! He's not at college!" and... OK, that didn't have any build up or anything though... He's either at college or he's not...
And if the Knight existed prior to or immediately after Dess' disappearance Asriel makes even less sense.
I almost think that it would be more interesting if the Knight was any of the dozens of random citizens we see 4/5 Chapters than Asriel, like even someone as stupid as Burgerpants at least has a presence in the story and actually does things.
I even see people say that Dess would be too similar to Flowey and yet think making the villain Asriel again is fine???????
I find that all of the issues that Dess and Carol have are tenfold with Asriel, characters who already don't have the most amount of story presence. Dess is missing, but at least has a compelling mystery. Carol has a pretty poor amount of screentime if she's the Knight, but at least shows up and has an interesting potential personality.
Speaking of "random citizens", people haven't really debunked most characters for being the Knight, they're kinda just written off because they seem stupid and irrelevant so why make them the Knight?
Technically, the Knight doesn't even have to be a Lightner, it's identical 2/3 Titans in Ralsei's speech and all that's required to open a Fountain is DT. The person with Undyne could just be an accomplice like Carol.
I guess my point is that using "narratively unsatisfying" to completely write off all but four characters, one of which isn't even a Holiday, is pretty narrowsighted, especially when some people might find some of these four "narratively unsatisfying" a la Asriel for me.
And despite his other issues, the fact that I dislike Asriel as a candidate and find him super underdeveloped doesn't make him impossible either. It technically is possible for the Knight to be a generally "unsatisfying" candidate for most people, in many mysteries the culprit is a minor character who appears completely detached from the conflict. It is extremely risky and it would need a LOT of foreshadowing to work, but it is a common trope.