They don't have to ask permission from the landowners, that's nonsense. It's Thorpe Park who usually have to ask Merlin HQ for budget and development approval, anything bigger Merlin HQ dictate the projects themselves (and design/deliver them with MMM) with consultation from the park. This is literally irrelevant to Bouncezilla.
Thorpe Park arent 'testing' anything new here, theme parks have been doing summer events for decades. It's an important part of the season cycle, it's just that Thorpe Park havnt been doing much in recent years.
Let's all just get our feet back on the ground. What are you basing this on? How would you even measure "worldwide theme park culture"? Do guests think to themselves "No I dont want a new ride, that's so 2002, I was a bouncy castle!" One doesnt replace the other.
The reason Thorpe park installed a lot of coasters in the early 2000s was to expand the park. After the buyout, it had to go from an independent family park into a big player for Tussauds, and that meant growing the market and doubling attendance. 3 big coasters in quick succession was their way of doing that and it worked, it changed the park.
Afterwards, it became a matter of sustaining attendance. Merlin's formula was a steel coaster every 3 years and go for IPs, horror and an 'Inbetweeners' type audience. Merlin realised it wasnt working, but spent years faffing around not giving the park the budget and cleanup it really needed. DBGT was meant to solve the park's problems but it was a spectacular misfire.
I agree that adding coasters every year is not necessary or sustainable, BUT you're talking an over-simplified version of events and pulling logic out of thin air.
Whichever way you cut it, the park's development these last 10 years has been managed very poorly. Its clearly put the park in an identity crisis and that's the bigger issue.