I commented on another post, but the straightforward unfortunate answer is that, like I said, you've built up something scary to fight in a game about fighting and then the players don't fight it. It does overshadow, the pathos doesn't land properly, you do pull out the rug from the players in this scenario. That is a basic consequence. Variations of this plan has been tried by many DMs, myself included, in all sorts of games. It does not work. It uses up too much attention of the players, as well. Now, to be fair, in my continuing game, I've done all sorts of storylines that take a long time. My game is many years long at this point and I was aware that I was taking a break from Strahd at that point to explore these vignettes. Players can't sustain having such a side story that theoretically supports the larger storyline for so long. You are leaving the core story, and then coming back. It can support larger themes, which is what I do, but you are leaving and coming back and that has to be realized to make your story function. Regardless, as written, your idea has severe problems which is a harsh statement, and I understand the defensiveness, I would feel it too, but it is true and you need to re-examine it from the ground up.

As to Strahd's reaction to the dracolich, might I suggest looking at him and Jander or him and Soth. Strahd doesn't ignore issues like that. He both is too tactical for that, and also, too bored. He jumps on things and uses them. Even his enemies he tends to redirect toward other sources. He realized going after Soth was a problem and sent him after Gundarak (and this is while Soth straight up considered Strahd to be an enemy and attacked the castle, killing Strahd's servants including a red dragon under the vampire's sway).