Dark rides do not remain 'star' attractions for long - unless they are changed every few seasons, and even if they are, changes do not have the 'pull' of a new coaster or other attraction.
I disagree, I believe that to be a mentality unique to the UK. Possibly because the dark rides on offer (besides Hex and arguably Sub Terra) aren't in the same league as those available abroad. A dark ride can be a headline attraction, a large part of the appeal in visiting a park, as can potentially a water ride. They just have to be well done.
Prime examples being Islands of Adventure's Forbidden Journey and Spiderman 4D rides, heavily advertised and quite possibly the best two known rides in the park. This is because they're of such significant quality and broad appeal that they're guaranteed to appeal, and they're marketed as headline attractions to boot. Similarly, the iconic attraction at Disneyland Studios Paris is undoubtedly Tower of Terror.
Fact is, Alton Towers don't market their 'experience' rides as headliners, and that's the main reason as to why they don't get particularly noticed, combined partly with their unwillingness to spend the same amount of money on a dark ride than they are a coaster, unlike these foreign parks. Nonetheless, considering how well-liked Sub Terra now is (in a way that Hex never really was, in terms of the general public), it might encourage more investment into this type of ride.
In my opinion, dark rides have more potential as star attractions than coasters; they boast broader appeal, generally superior throughput and a changeable, perhaps more re-rideable, experience which will retain popularity if properly built, advertised and maintained.
The UK needs one of it's parks to take a risk and spend a decent amount on a dark ride. Not necessarily Alton though, and not necessarily a Duel replacement. Alton should have higher priorities than changing a ride which remains perfectly decent.