I don't do them solo much so I can't speak for the crowd that loves this, but there is definitely a dedicated niche for that and I can see why, there is a lot of decisions and things to manage carefully while running them solo, and it's unforgiving for the people that also look for a challenge.
I do agree that the leveling sections are boring and tedious, and that's exactly for similar reasons that normal dungeons can also be a snoozefest. I do however enjoy the semi casual experience they offer in optional higher floors.
I also do agree they need a shake up to make them more engaging, but this cannot come at the cost of their identity and niche which prompted this thread.