One way or another the park does seem to have stagnated. Although Thomasland seems to have done very well, it doesn't appear to have given the park the same boost that Peppa Pig gave Paultons. In 11 years they added Splash Canyon, Shockwave, Klondike Gold Mine, The Haunting, Sombreros, Storm Force 10, Wild West Shoot Out, Apocalypse, Maelstrom and Excalibur. In the 14 years since then it doesn't really feel like we've come that far forwards. I think it's a bit harder to see why.
It's unfortunate that Excalibur didn't work out for them. I actually thought it was a decent ride and it was certainly different to anything else in the UK. I can't imagine that they let it go lightly so perhaps they spent quite a bit trying to resolve the problems with it, before writing it off. It's also a shame that they had noise issues with the Enterprise they bought. They probably burned their fingers twice in quick succession with Excalibur and the Cyclone. They also tried a Christmas event in the early 2000s that only ran for one year, so presumably that didn't work out financially.
Ben 10 didn't seem to work out as well as they hoped either, as they didn't develop the Cartoon Network Street and Ben 10 lost its IP after 5 years, which wasn't very long. Realistically Ben 10 wasn't a massive investment by modern standards (about £2.5 million I believe) so there's a limit to how successful they could have expected it to be. I don't know whether the issue was that Ben 10 was the wrong IP, or whether they expected too much from a coaster that all said and done wasn't that big, and where the theming was patchy. If they could have spent another half million to properly enclose and theme the station it would have felt like a more complete experience. Perhaps Ben 10 was a big enough investment to leave a hole in their pocket, but not big enough to bring in many people.
It's also a shame that Pirate Adventure was allowed to decline to the point where it was closed, as it was one of the UK's biggest dark rides. Having Pirate Adventure and Excalibur both closed does leave a big hole down that side of the park.
I don't think any of this really explains it though. Did they borrow in the 90s to expand, and has their debt caught up with them? Or did building the hotel stretch their finances and not bring in as much income as they anticipated? Did the Smiler accident hurt DMP's visitor numbers? Did competing with Merlin push them into a discounting model that hurt their bottom line? With Drayton Manor I don't think there's anything really significant that went wrong for them, aside from the Splash Canyon accident. But things had stagnated before that. It does feel like there might have been a series of events that slowed their growth.
Things do need to be put into perspective though. Losing Excalibur and Pirate Adventure does leave a big gap. If Splash Canyon doesn't open next season that's another major ride closed, and losing the big wheel, chairlift and buffalo really do take out another chunk. Three out of five sides on Apocalypse not working is also worrying considering it's one of their top rides. The park's starting to feel a bit empty in places. At the same time Thomasland has genuinely been a big development for the park, Ben 10 isn't a bad little coaster with a nicely themed queueline, the park has a bigger entrance area compared to 2003, Pandemonium is a nice step up from the Sky Flyer and hopefully a hotel will in the long term be a catalyst for growth. I still like Drayton Manor. I still think it has some creative rides, and I can still have fun there. It just needs bringing back up to the boil.