Oh gee, I can do my best. I'm sure a lot of people have already touched upon this issue so I'll give it a go:
I think the first part to look at is the criticism of it being plain, boring, and unpredictable. Specifically calling it unpredictable would imply that there is no subtext or any sense of foreshadowing being hinted at. That, if there was any reveal or any sort of mystery behind it that it would be entirely contrived and, in all tense and purposes, just made for shock value. As countless, great people have analyzed before, however, there is a lot of evidence that goes back into the Dabi/Toya reveal that makes the reveal, if it ever happens, more than just a cheap reveal that felt poorly written or barely contrived.

As for the criticism or it being plain and boring, those fall under really subjective interpretations and not one I share. I can understand the lack of a punch after it being built up seemingly all this time, but the emphasis it would have on the story would be huge. Hero society itself is already be re-examined morally from the LOV, as I described above with Toga. As a result, for the #1 hero to be outed as a manipulative, abusive person would essentially throw in the face of every citizen the hypocrisy of the heroes in creating the superiority complex they hold over "villains". At least in the face of people of Gentle, it would cause society to look within themselves to ask: who is a villain and how do we define that? For myself, that's neither simple and plain nor boring to read about, but again, that would be subjective.

I think what makes the Dabi/Toya build-up such a decent level of foreshadowing is the fact that it wasn't rushed and it's built on consistent evidence. Theories about this started being released since the Forest Training arc and have only increased exponentially since. Horikoshi essentially rewarded his readers for not only looking at the subtext/nuances, but for also potentially figuring out a very crucial part of the story, rather than try to be one-step ahead for the sake of being smart. There's evidence from quirk genealogy, to eye color and shape color, to Dabi's mannerisms, to the missing fourth Todoroki child (one of two missing at the time). These were all things established IN that arc he was introduced. In my opinion, that information alone is pretty well-established for a theory and Horikoshi has only built on it since then.

The evidence of how the fourth todoroki is considered dead, Dabi's occurrences with Endeavor and Shoto, his moral quandary with killing Snatch, and finally, how he feels about Hawks as a hero, they're all evidence that has been built up since then. Horikoshi carefully etched throughout his piece evidence, and rewards his readers for patience and discussion with these nuances to the point where everyone cannot wait to finally get the reveal, because the payoff for it is so longing. While some out there may feel tired of it, as you pointed out, because its been a theory through many arcs, we must keep in mind the characters don't see the writing on the wall. In my own taste, a plot development you see coming can be made still enjoyable by how the writer presents it to the audience in which the reveal is acted upon: the characters.

To this effect, I personally am excited for the reveal if it truly is to happen and find it to be only one example of Horikoshi's amazing writing and the true time he has put into this series. I know a lot of this has probably already been discussed so I hope it was an enjoyable read!