Fixed this for you.
I think it is arguable to say that Hex remains Alton Towers' headline dark ride. The Curse has the potential to take that title, but this won't become apparent for at least another season or two once the initial hype has died down. Outside the rollercoasters, the park's 'headliners' work in a rather different way, which means rides like Hex and the Congo River Rapids also remain headliners many years after their addition.
My mistake; I think I phrased that badly. My point was that from my experience, I don't think Hex is a headline attraction for the bulk of visitors to the park, so I'm not sure that Merlin would view a full replacement as viable relative to the amount of money it would cost.
Out of interest, what metric are you using to classify Hex as the park's headline dark ride? If it's based on presence, I'd argue that Curse now has a very legitimate claim to the title seeing as it's the one that's currently plastered all over ad campaigns and widely talked about within the public conscious.
From what I can tell, I would struggle to argue that anything that's not a roller coaster, with the exception of CBeebies Land, is a headline attraction of Alton Towers these days (in my view, of course). Rightly or wrongly, I think the park is mainly known for its roller coasters among the wider populace, and it is perceived as a roller coaster park first and foremost. If we were talking about Disney or Universal, or to a lesser degree somewhere like Europa Park, my view would be different. But at Alton Towers, I would personally struggle to argue that anything non-coaster, bar CBeebies Land, is a headline attraction at the park these days.
With this in mind, I am unsure whether Merlin would view a like-for-like refurbishment of Hex as viable in the context of modern day Alton Towers. For the money it would cost, would Merlin deem it to have satisfactory returns? I'm not so sure.