There is the old idea that sequels aren't as good as the first, which is true sometimes and not true other times.
True, but my point was that even if the sequel sucks compared to the original, there's no reason to be mad about the fact that it
exists, as though that somehow ruins the original.
You make an interesting point and one that is often valid. However, I think there is some precedent where sequels did, in essence, ruin the original.
Alien3 is possibly the most glaring example of this. It basically took what everyone loved about Aliens (Ripley's heroic rescue of Newt and triumph of the Aliens) and gutted it by killing Newt and revealing that Ripley was, herself, infected. The sequel meant that everything Ripley accomplished in the second film meant absolutely nothing.
Another example could be the "Rocky" movies. The first film was a serious and Oscar winning film about a down on his luck loser who gets a big break, loses, but gets the girl. With each sequel the series got more and more cartoonish until people all but forget how good the original was.
I also think people sometimes get made about sequels, not because they ruined the original, but they effectively prevented better sequels (or at least foreclose potentially more satisfying story lines for sequels).
In this category, I think you could include Superman Returns. Singer had an opportunity to reboot the Donnerverse, keep the stuff that worked (the Williams score, the Reeve style of portraying Superman, etc.), jettison the junk (Lex as a real estate swindler) bring it into the 21st century and wipe the slate clean of Superman 3 and 4. Unfortunately, he was overly slavish to the junk and then he threw in a plot "twist" with implications that made the idea of sequels unpalatable (basically, the kid, the idea that Richard White was the actual good guy in all this, Superman abandoning the kid AGAIN, etc.). As a result, we had to wait years for yet another 'reboot' Superman movie...one that was, itself, flawed.