People say that there have been “15 years of neglect”, but from what I know, I’d argue that the neglect could well have started even before the Merlin era, as I gather that Tussauds, particularly the later iterations, were not exactly known for maintaining things either.
Just a few years into Merlin’s tenure, pretty much all of Chessington’s original theming needed tearing down for H&S reasons. That level of rot would not have set in during Merlin’s 5 years or so in power at Chessington at the time, and I’ve even heard it said that DIC very nearly sold Chessington prior to the Merlin buyout due to how decrepit large swathes of the park were and how excessive the cost of replacing the unsafe theming was.
I’ve also heard tales of how The Haunted House allegedly declined in quality and maintenance as early as 1998, and how some of the dark ride refurbishments of the 2000s, such as Duel, Charlie and Bubbleworks mainly happened due to the original rides being undermaintained.
I fully agree with your general point, but I’d in fact argue that the “neglect” could have started earlier than Merlin’s tenure. I would argue that Tussauds simply had an easier time of not maintaining things than Merlin because a lot of the core infrastructure was much newer, and during the 90s at least, there was more investment into new attractions and more “medium” years.
If you think about it that way, we could in fact have had 20+ years of “neglect”.