That's your opinion. For me it's just an overplayed trope and it's been kinda brushed under the rug after the reveal with nothing really coming from it and the plot feels more muddy and unclear where it's trying to going now.
Elizabeth turning out to be a fucking unit and not a fragile doll was so much more interesting than the "real Ciel"
I don't really see how the author "forgot" her. Yes she hasn't shown up for YEARS, but due to the extremely slow updates and short chapters, that's just how it is. In the story, I'd say only a few days technically passed from the last time we saw Lizzy. We have about 4 events happening all at once and we can only show one at a time. I'd say just let the manga marinate until the very end so you can read it all at once instead of waiting for bits and pieces for years, and the pacing is a lot more bearable this way. Though I do think it would be nice to get little chapters here and there updating us on what the other characters are doing since our crew split up

I used to follow this since the beginning but after the twin reveal arc, I stopped reading for a bit to let it build up. But I forgot to come back and now I have no clue what is even happening anymore atp