It's not even a theme park fatigue. You've just grown out of theme parks.
Thats the rose tinted glasses. Back when Dragons Fury opened, the period between 2005-2009 was its darkest days. The only new addition being Peeking Heights but we had Bubbleworks trashed, theming ripped off Dragon Falls and minimal investment across the Zoo. Whilst you say a handful of re-themes, we've seen refurbishments of Flying Jumbos and the Berry Bouncers, Runaway Train into Scorpion, Bubbleworks into the vastly superior Gruffalo, Buccaneer refurbished, Dragon Falls refurbished, re-themed and new systems in place this year, Room on the Broom and the Zoo area significantly expanded and improved. Wanyama Village is one of my Chessington guilty pleasures. And finally, I didn't realise this that Chessington has the longest season openings of them all with Winter offerings (Xmas 2018 had Vampire, Seastorm, Gruffalo and Buccaneer available).