I think maintenance does make more of a difference than that. How popular would parks like Europa, Disney's and Universal's be if they were all falling to pieces?
Though maintenance won't lead to much straight away, maintaining the park's older attractions and occasionally updating them (if they need it) is going to leave you with an attraction that's going to be much more appealing. If you're having a better time you won't mind spending a little extra, but if you pay as much as it costs to enter some of the parks here in the UK and feel cheated because the park isn't what you expected for the amount it cost to get in, why on earth would you want to continue giving the owners money?
I had no problem buying merchandise at Universal Orlando because I wanted to remember what an excellent time I had with a few trinkets, but I spend as little as I can at most parks in the UK because they don't offer very good value at all.
Big companies are only interested in ways they can make as much money as possible whilst putting as little in as possible in the shortest amount of time that they can. We all recognise how important upkeep is and I'm sure those at the parks do, but those in-charge of the funds won't care about anything other than saving and making more money. Who cares if the Nemesis alien looks like crap when you can just make another big ride with the odd piece of theming and a few more inversions on the other side of the park to distract people?
Meh. :/
