Old Forbidden Valley (after Thunder Looper was removed) had a theming structure near the entrance rather than a civilian helicopter, a theming structure behind Ripsaw over nothing like we have now, red waterfalls flowing into a rock pit, and at one point had a themed attraction where a corrugated shed now sits. It was just rust and (dirty) rocks (and weeds) for the latter part of its life.
I visited Universal Orlando (wasn't called that back then), Disney World, Sea World, and Busch Gardens in 1994, less than 2 months after riding Nemesis for the first time. Sure, there was loads of fiberglass and monorails going through hotels in the gun toting US of A, but Nemesis felt like it was world class to me at the time.
But even then, it was nothing like Rookburgh or Klugheim are now. The industry moved on in the 2000's, Tussauds and Merlin didn't. They have never built an area like either, and to suggest they would out of the blue would be out of character for them. Imagine if the new Forbidden Valley was of Phantasialand or Europa quality? You'd walk onto Walliams World with it's flaking primary coloured paint, Dark Forest with it's prehistoric theming painted grey, and Katanga Canyon with it's absolute nothingness, and wonder if you're in the same park.
It's an amusement park next to a gothic mansion in the woods with a few high quality theming elements, like the Wicker man structure and Nemesis creature. The likes of Phantasialand, Europa, and Efteling are theme parks. Towers isn't. The comparisons aren't valid.