Is Hex a headline attraction, though? For the average guest, I don't think it is. I won't deny that it does very well in the role of a filler attraction to ride between major coasters (a category that is increasingly lacking at the park), but I wouldn't personally consider it a headline attraction with enough of a pull to warrant spending millions on a hefty refurb job.
I certainly think they could do a refurb (there's not really much else they can do with the space, and it's generally well liked), but I'm not sure it would be as much of a priority to park management as a major coaster like Nemesis.
I know a lot of enthusiasts like Hex (myself included), but I think it's stock among the average guest is a lot lower; most non-enthusiasts I know find it boring, and I don't think that the majority of Alton Towers visitors would view it as anything more than a filler attraction.