To nitpick, Sub-Terra was not advertised as 'new for 2023'. Apart from some social media mentions, it was not advertised heavily in any way. On the other hand, The Curse was advertised as new with national advertisement and pushed heavily across all different types of media.
The Curse was a substantial retheme and refurbishment of an existing ride. Sub-Terra had minimal work and is effectively the same experience it was in 2015. There's a big difference between the both.
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It makes sense to postpone Horizon if flat rides will be added in between. I know the argument that flat rides do not have broad appeal is brought up, but Alton Towers is good at marketing, and 2 or 3 new rides advertised in one year will have some appeal. Let's not forget that Alton Towers has had many 'low budget' years where smaller attractions or area rethemes have been advertised. These all have less appeal than a huge multi-million-pound coaster, but they still draw appeal as a 'new' experience.