These things don't happen at most major theme parks.
I fully support what you have said about the individualised reasons behind these rides requiring additional work, but surely they should be able to deal with this - in any project management, you always allow for buffer periods in case of need. It is common. Rides require maintenance or refurbishment or a strip down and re-build. It's part of the deal in the technical aspect of running a theme park.
If I went to Alton Towers this Saturday and 25% of the rides were closed (which was the case for most of Saturday at Thorpe Park) then it would be the poorest ride availability the theme park had ever seen. But with Thorpe Park, it's almost like it is to be expected.
Why should it be expected? It is not like this is a one off - it has been happening for years.