This feels like a misrepresentation of what was going on at the time.
Whilst I would agree that the reaction to Duel being announced was fairly muted and there was “no great outpouring of loss”, the news that people were reacting to wasn't that The Haunted House was closing. The park were very clear that what they were doing was revamping/upgrading The Haunted House, which most people agreed was much needed. There is a reason that Duel has the tag line of "The Haunted House Strikes Back" - this was the park emphasising that the attraction wasn't going anywhere.
Also, you have to bear in mind that at this point Tussauds were well known for adding high quality attractions - they had very few misses over the previous 15 years, so if Tussauds said they were upgrading something, it was broadly expected that it would be good. To be honest, it is almost the inverse of where we are now with Merlin, where even if an attraction sounds good on paper, you almost have to expect it won't have the budget required to do it justice.
In contrast to the community's reaction when Duel was first announced, the reaction to the ride when it opened was pretty poor all round. Even the reviews that rated it as good still highlighted how bad it actually was. When discussing the new zombies, a four-star review at the time included the line: "Frankly, I expect better animation from a pop-up book." And lots of those criticisms from day one are essentially the same ones being discussed today.
It is worth pointing out that one of the recurring features of those early discussions was just how bad Duel's lighting was in comparison to The Haunted House. I don't think it can be stressed enough that when you are riding Duel, whenever you can see either the ceiling or walls of the building, you would not have been able to see them in the Haunted House. The lighting was so much better designed and so much darker that essentially you only ever saw what the ride designers intended you to see. And if there was any chance that the riders might see a bare wall - it was themed, so it blended in.