I understand what you are saying and I never rode the original haunted house however I do think something will be done to duel at some point. Whether it's returning it to the haunted house (I agree it's unlikely but cool to speculate about or something else). I think the park must know that something needs to be done to it and you have to remember that Merlin is made up of people some of which will care and do care about the rides themselves. I wouldn't be surprised if the creatives have tried to do something to Duel but have been told no by the board. Merlin has changed hands recently and it has gone private. We don't yet fully know what there approach will be however it looks more promising than later PLC Merlin did. I hope it signals a shift to ambition and long term planning and investment like the earlier Tussauds as we know it works because you can just look at the attendance numbers over the years and see what the public prefered. Alton Towers need to sort out its selection of rides for the whole family and that means making sure there haunted house is the best it can be.Honestly... I think a lot of the love for the Haunted House is overstated.
I was on ATA and Park Life (which was the TowersTimes forum before TTF) in 2002, and had my first visit that season. There was no outpouring of loss for the Haunted House at the time. It was generally seen as half broken from what I remember (although it was certainly a lot less broken then than now). Large stretches had nothing on them at all where failed effects had been turned off.
It certainly wasn't a particularly popular ride at that point either.
It was only in the years that followed that sites like HauntedOne turned up, which did an excellent job at explaining what the ride was supposed to be like at its very best. I don't know that it ever actually was like that though, and the fact the ride was twiddled with almost every year for the next 10 years suggests to me that if there ever was a period that it was brilliant, it was shortlived. Whatever the accepted reality is nowadays, the fact is that many of the effects of the original ride were removed or changed - in other words failed.
I can't see a full scale internal redo ever happening. It's too big to be palatable, when you look at the size of the Gangland Grandmother ride on Cred Street you see how far ambitions have come down since 1992.
Better to look forwards than back even if it was on the cards.