If it was mostly nostalgia then Superman 4 would be well regarded also.
STM is a groundbreaking classic. This iconic dialogue, music and scenes, Reeve's performance, the flawless origin story, the optimism that captures Superman's character, etc.
It doesn't need rose colored glasses.
I mostly agree. I wasn't referring to Superman: The Movie specifically, but more about why people wouldn't admit to simply liking something based on nostalgia alone if they wanted their opinion to be taken seriously.