Here's my take on this.
Maybe some of the issue is that the word "nostalgia" is looked upon as a bad thing to feel for an old school ruleset. But it's not.
As someone stated upthread, Goodman Games used nostalgia for their marketing and it was wildly successful. It MUST have some kind of effect, mustn't it? I mean if it's used as a marketing angle and the angle delivers, then there's some nostalgia out there that can be plumbed for sales.
Nostalgia for a ruleset is not a bad thing!
My buddy has a late 60's Camaro SS, super sweet condition, black, all the chrome polished and it runs perfectly. The motor growls when it idles and it roars like a beast when you punch it. It drives like a DREAM and the rush you get when you plant the pedal-to-the-metal is insane. It's an awesome car. Now. Today.
But we drove into the supermarket the other day and an older guy, who must've been in his mid 20's during the late 60's came over and asked about it. He had one back in the day. He ran his fingers across the lines of the car and mumbled how awesome it was. His eyes glazed and he smiled like a kid.
He was remembering the car in the same way that my buddy and I were experiencing it today. But he was most definitely nostalgic about it. Was that bad? Of course not!
I would venture to guess that if something gives that kind of feeling, today, and yet it's heyday was 20+ years into the past, most people that go back and re-experience it after a time away are doing so with some nostalgia, whether they know it or not. It's not a bad thing. Note that I said MOST, not ALL. Of course there will be those that say there's no nostalgia. It's just a really hot car.
OK, that's cool.
But, please, be honest about the fact that there's likely another, possibly much larger body of people that DO feel the nostalgia and that's why they give it a try again.
Nostalgia exists and is used to guide our purchasing habits for a lot of things. RPGs aren't much different.