Yes, this is true. The reasons include:
-A decrease in visitors compared to 2011. Despite the fact that 2011 was one of Thorpe's busiest and most profitable (and so, in that sense, 'best') years ever, Swarm still had to deliver an increase. It didn't, so Merlin saw Swarm as a failure, and immediately lost faith in whatever their thought process was behind that direction.
-Weak brand. Swarm didn't capture people's imagination. It was very generic and not very recognisable. The end of the world vibe was overdone and it merged in with so many other things that went for that.
-Experience didn't match marketing. For one reason or another, people expected an intense thrill ride, similar to Saw. Instead, Swarm offered a much more graceful experience, leading people to be disappointed.
So basically, people had minor niggles about the ride, and less people visited Thorpe (which would have happened regardless). So, Merlin see it as a failure. Despite that, in the long term it's done a good job, is less generic these days and scores very highly in KPI scores, which Merlin love. So it's been a slow burner. But Merlin's model doesn't allow for slow burners.
Just a couple more to add:
Derren Brown's Ghost Train
Going for a dark ride over a coaster was always a risk, but it was one which needed to be done in my opinion. Sadly, DBGT missed every single possible mark, which really damaged the park. It did attract attention, and it did cause attendances to rise. But it damaged the park's reputation by so much more than the worth of a few thousand visitors. Two big investments in a row that failed is a big recipe for disaster. You need a sure-fire win after that; something with minimal risk. But when everything is spiralling downwards, the pressure is even great, and the amount of risk you take has to decrease. Ultimately it leaves us in the situation we're in now: the best way to minimise risk is to not take a risk at all. Can't lose money if you don't spend money, right?
Terrible Non-Big Investments
You have to go all the way back to 2007 when the park added a solid non-big investment, in Flying Fish. Arguments can be made for Storm Surge (diversifies the line up and added a much needed family ride) and Walking Dead (a solid scary-themed indoor attraction at the end of the day). I guess you could say 2013 with turning X forwards but that slipped under the radar with the focus on Swarm backwards. But I think Flying Fish is the last one where someone would struggle to come up with a reasonable negative for it. Since then, we've had short term investments, based off fads or needless IPs, which have led to a mish-mash of identities and ideas, and nothing which has really improved the line up.
This is coupled with a lack of general upkeep in the existing parts of the park. The bare minimum was done to rides, theming, presentation, etc., which eventually led to rides closing, theming being taken away, etc.
Focusing too much on Scares
Fright Nights works for the park. Regardless of its quality, it's still the busiest, best received, and most profitable time of year for the park, year in year out. The park tried to capitalise on that and make it work all year round (which, in fairness, isn't too unreasonable). It worked with Saw - The Ride, because that was handled properly. Saw Alive was a nice idea and helped absorb the busyness whilst it was full time. After that, Thorpe should have quit whilst they were ahead.
Scare attractions don't work outside of Halloween in a theme park setting. Scary stuff doesn't work outside of Halloween in a theme park setting. One thing to compliment a line up, yes. But an overarching focus on it? No. Yet Thorpe didn't realise this. They didn't learn that in 2005 when Freakshow 3D was dead during summer because no one cared. They didn't learn that after Saw Alive's popularity dwindled in 2012. They didn't learn after Swarm. They didn't learn after Ghost Train. They probably haven't learnt after The Year of The Walking Dead.
Keep scares for Fright Nights, and focus on a diverse line up.
Not Learning Lessons
Related to above, Thorpe make the same mistakes again and again. This likely comes down to management seemingly being a revolving door, especially in more recent years. A new person comes in, tries something, it doesn't work, leave. A new person comes in, tries that same thing in a different disguise, it doesn't work, leave. Repeat. They need to take a step back, look at what they've done for the past 15 years and realise what hasn't worked, and what has worked, and start building from that.