The Disney / Universal are the biggest, wealthiest entertainment companies on the planet and are also in competition with each other, so I wouldnt say that's a fair example. It needn't be as good standards as Disney, it just has to be decent.
A better comparison would be the likes of Europa Park, Efteling, Phantasialand, which are all run by companies infinitely smaller than Merlin, but they maintain their parks fantastically. A trip there costs you about the same as a Merlin park but is infinitely better value.
True it's not Chessington's choice that they get no budget or structure for this kind of maintenance, it is Merlin's method. Like you say, if it isn't able to be rebranded then they just don't care, this is their strategy. Even with new rides, once the press launch is over they rapidly stop caring. It's not the attitude of a real entertainment company interested in return customers and long term benefits for their parks.
However, Chessington is particularly bad at maintaining its effects and attractions, compared to Alton Towers for example. Sometimes it feels like, top down, Chessington are totally unaware that audio, lighting, etc even makes a jot of difference to guests.
You would very soon notice the difference in guests' reactions if these attractions as good as they were originally. The fact that "not many" of them actively complain about specifics is true but not really a valid reason to not maintain something. Why should it be the guests' perogetive to know the details of lighting, sound, animation, etc? They are paying for a good experience and instead getting a sub par overall experience.
IL Bubbleworks had plenty of regular complaints about how poor it was long before it got removed, nothing was done about it for years until they could get a new brand in. Also, the fact the park looked shabby and run down for years was a frequent online complaint from your average guests, but all that happened was they got the local painter decorators in to paint one facade per year.
The only excuse Merlin have is they can just hype up the brand and monopolise the UK market so nobody knows the parks could be any better.