Overall I do think the Tussauds annual pass is what caused some of the issues for Chessington as it probably drove more people to the park compared to day tickets, particularly repeat visits by the same family.
For a small park with a zoo it was pretty good, but I expect the influx of passholders in the early 2000s just pushed too many guests into the park.
I do think there’s something in this.
In 2008, park attendance rose by
29% (~968,750 in 2007 to ~1,250,000 in 2008, as per Merlin’s attendance graph) despite the lack of a major new addition.
Merlin gained control of the parks in summer 2007, and 2008 was their first “proper” year in control of the parks where products like the Merlin Annual Pass properly entered the public conscious.
With all of this in mind, I’d wager that the MAP was at very least a significant contributing factor to Chessington’s rise in attendance in the late 2000s and 2010s.
However, I don’t think it tells the whole story, nor do I think it explains why Chessington struggles so much with queues nowadays. The park maintained a peak attendance level of around 1.8 million between 1995 and 1998 (a level that has never been hit since, even under Merlin), and anecdotes from the 1990s suggest that the park never suffered with long queues quite like it does now.
From what I’ve heard, I feel that the issue can be blamed more on throughputs for major attractions dropping due to H&S requirements, although the guest figures rising again in recent years certainly won’t have helped.
EDIT: Sorry, I wrote this post before your most recent reply…