Yes. Though I think the concept of Superman was already very solidified in the public consciousness by the 80's, I do think Donner and post-Crisis helped refine and harden the public's general idea of Superman, and it's since turned into something so firmly entrenched in our culture anything else seems off to the casual viewer.
The Golden Age was so different from what we've had since the 50's I think that would be seen as jarring no matter what though.
Well, it's comics. It's *all* superficial.
Absolutely. If the public refuses to accept any other version of Superman, and the version they've latched onto is actually among the worst options out there (which it is, for lots of reasons) then yes this is detrimental as hell. It means that they're not going to easily accept anything else, like a kid who refuses to take their medicine.
I hate the term "classic." How are we defining it? The oldest version? The one stuck around the longest? The one that's most well known? Those all have different answers.
I don't think there's a specific version of Superman that is the "classic." I mean, people might think of post-Crisis or pre-Crisis as "classic" but there's different mini-eras in both where the character is completely different (compare Byrne's Superman to Loeb's; very different people) I think the "classic" is a mishmash of everything; all the random stuff floating in the Super-verse that most people would point at and say "That's him."
Speaking personally, I think Morrison probably did the best job of creating a "influenced by everything, it's all in the mix" Superman. His meta-Superman arc (JLA, Action, All-Star, 1 Million, etc) is probably the most brilliant specific version to see print (only because I havent had my turn yet! /s).
I grew up on the triangle era and was a hardcore post-Crisis defender for years. When the quality started to drop in the late 90's, and then again in the 00's, I had to go to older material for my Super kick. Which is where I discovered the Golden Age (my favorite era ever!) and everything else. I also learned that almost everything I thought I knew about Superman was, at best, a half-truth and at worst a full-blown detour in complete opposition to how the character had been built.
I think we're all far too concerned with where Superman has been, and not nearly as concerned as we should be over where he's going.
We all have our preferences. And there's nothing wrong with that. I lean heavily towards pre-Crisis trappings and am a very big fan of the "mixture" approach they're taking these days where they try to include both pre- and post-Crisis elements. But someone who's only into post-Crisis, or something else I'm not big on? Fine. I'll disagree with their opinions but they're allowed to have their favorite take too. We're all fans. Even if we're fans of different eras we're still all Superman fans, we all want to see the character do well.