I think the investment needs to be taken with a bit of context.
Thorpe do have a decent range of coasters for a park of their size. Not perfect, and in this day, not as varied as others, but still decent. Around 2014 (when planning for Ghost Train was in full swing), plenty of people were crying out for something different. And Thorpe needed a dark ride.
It was still a huge risk to opt for a dark ride over a coaster, and one that in hindsight really hasn't paid off. But at the same time, when this was announced and being constructed, the overwhelming majority thought it was a good idea.
The issue isn't that Thorpe didn't build a coaster, the issue is that they built the wrong dark ride.