The Ghost Train investment was realistically always going to be high risk. It was being developed when consumer VR was in its infancy, and they've essentially just used consumer grade technology in a high stress industrial environment. It would be incredibly disappointing if Merlin have got cold feet over spending anymore money on Thorpe because of Ghost Train, given they would have known the risks involved with the project from the very start.
It's hard for me to see what Merlin are waiting for, in terms of investment into Thorpe Park. From when they were public, the accounts showed they were vastly profitable as a company overall, and were happily building more and more things in untested markets, such as China. So money clearly isn't an issue.
My guess is park attendance increased in 2019, which given next-to-no investment is a remarkable feat. Although, perhaps reducing ticket prices to possibly an unsustainable level had a big part to play there. But it indicates there is a healthy market there, which given the park's proximity to London and one of the world's largest international airports, shouldn't come as much of a surprise.
Without some form of investment, even to ensure the park remains in pristine condition, stagnation will be joined by disrepair. Any hard work Thorpe have done to improve guest numbers and satisfaction will steadily be drained away by Merlin's unwillingness to spend money, making obtaining a healthy, self-sustainable and highly profitable theme park, ever more difficult.