A simple, lettered complaint won't necessarily do anything - certainly if it's 'preachy', it must not be.
The only time we had successes with this sort of thing, was back when we operated TowersTimes, and conducted a thorough petition - however, this was based on a specific agenda; most closely linked to the Fanta situation on Oblivion, which was used to form the backbone of the argument, whilst fanning out to other significant issues of a few years ago. Any letters written need to be focussed principally on one or two issues - a general letter that covers too much simply will not be addressed (complaints about the entry price are frankly pointless).
A simple "it used to be like this" just does not work, however it is worth noting that Alton Towers' general customer feedback (KPI) scores are quite poor this year, which is telling in itself. They are lower than those of Thorpe Park. Worrying, isn't it.
I frankly believe the park are well aware that they're doing badly at the moment. However, they don't seem to be able to do anything about it, because they're part of the conglomerate of Merlin Entertainments, which, as has been said for several years now, is holding the theme park back from what it could do - with exception to the addition of new rides. Yet, we all know that a theme park is not simply about new rides - look at the Disney model, for example. There are indeed new rides added every couple of years, but on other years - look what happens. Entertainment is focussed on (Disney Dreams etc.), ride refurbs (Test Track etc.) and food and beverage improvements (EPCOT's new restaurant in Morocco).
For as long as this great theme park, which has been around far longer than Merlin Entertainments, and shall be around after them too, is owned by that office in Poole, and controlled by those investors in the USA - we shall never see it operated to its potential as one of the greatest days out in Europe.