The big advantage of Sub Terra is that is is not a roller coaster, it is a dark ride, experience, indoor ride and it is unique, doesn't need an IP to sell it (OK it is piggy backing off Nemesis) if anything is shows how original Alton Towers are. I guess the people who dislike ST also dislike Hex?
The one thing Alton Towers brings to the table is variety and fun for all the family, all ages. It is disappointing to hear all the animals will be going this year, except Sealife, but they can easily get the back.
If Alton keep on adding unique attractions, I can see in 10-15 times it being a much superior park to what it is now.
What Sub Terra lacks, is the re-ride appeal. We all will go on Nemesis as many times as we can, but Sub Terra, you do it once and you may not even do it on multiple return visits. For those guests who visit once a year or less, they will go on it and enjoy it, so there is still plenty of people who will use the facility.
From a Merlin point of view, it does require a lot of staff to run it, in the long term, the running costs could mean we see later openings for the ride, or even it restricted to only the busier days if Merlin start doing cost cutting. Even if it became a part time attraction, for example half term/easter/summer holidays and scarefest, it is still a good addition to the park. I would welcome a reduced opening hours rather than removal if it ever came to that.
If you go down to the woods today