I think it's clear the numbers reflect on the things the park has done, and the path it has unfortunately strolled down in recent times. The 2010s are by far the most interesting, as we haven't got the crappy DIC acquisition period, and the weary Charterhouse period beforehand.
2010 wasn't a bad year actually, all things considered. Aspects of the park were beginning to age and we all know problems were rife as they had been for a long time, but not to a huge extent; much of the former park from the 90s still existed, fireworks returned, a brand new family thrill coaster was unveiled, and the monorail featured all of the park characters.
2012/13/14 was the major disappointment. The terrible gimmicky imaging (vekoma track nemesis being the prime example) started to appear, fanta.. well.. yeah..., ill thought through new attractions like Sub-Terra which in concept was awesome but needed far too many adjustments and would of been better if the focus had been more on the ride itself than a walkthrough. And Smiler, wow, something that could of been one of the most incredible projects the resort had ever made, a secret government themed ride that would influence how you think and feel.. but no.. the station was the first major let down. It could of had government labs and an introduction to the purpose of the X sector area and what you were about to experience. Instead it just featured a few illusions on walls and flashing lights. -_- The first section of the coaster could of at least had more effects and the shop above should of had a glass floor so that you could see the trains pass below. Nope. Again, signs that the park wasn't making room for visitor experience and was rushing major projects. The queue line... well, do I need to say anything.. everyone knows it's just a black metal fence zigzagging in the most boring way possible, enough said on that.
There was a clear lack of maintenance too during these years, and as many have observed, what little was left of entertainment was slowly eroded in to nonexistence.
THIS, in my opinion, is reflected by the dwindling guest numbers, that weren't enticed by the park or its new offerings and would of subconsciously been put off by the lack of experience compared to other major parks.
2015 was just the nail in the coffin, with the accident happening when it couldn't really get much worse. And look how atrocious those visitor numbers are, admittedly not always the parks fault due to the rags sensationalizing everything, but still bad.
Look at the number increases on the TEA for Europa park up to the near present.
3,950,000 4,000,000 4,250,000 4,250,000 4,500,000 4,600,000 4,900,000 5,000,000 5,500,000 5,600,000. Far, far more consistent. And this is as more themed areas are made, more attention to detail and visitor experience are offered, and regular maintenance has occurred.
One other thing, 1994 seemed like a fantastic year for attendance, and look, Alton Towers were doing things RIGHT that year. Some of the o̶l̶d̶e̶r̶ wiser members here would probably know what a year it was, but sadly neither of us were part of that generation. It makes you think, if Merlin genuinely think that park maintenance, entertainment, and sustained theming are just to cater for enthusiasts and won't reflect on the GP/guest numbers, then they will learn the hard way that this is not true at all as the stagnation will continue, regardless of pricing and new accomodation. What a pity AT didn't learn from their achievements; we can only dream that we can go over the 3 million mark again.
