I think your math is off. Joe Quesada is only three years older than I am, and I only was reading comics for a couple of years before Gwen died. (and I was younger than average when I started) Granted, I managed to read a lot of back issues at the time, and your larger point still stands because if you asked me who the woman is in Peter's life, my instinctive reaction would be Gwen. Besides, it's impossible for someone to have grown up with 10-20 years of Gwen being alive, since she was only alive for 8 years.
There's a very small window for someone to have grown up just knowing Gwen as Peter's girlfriend. I, and presumably Quesada as well, was there for the transition to Mary Jane, so it's just as reasonable to me to see Peter with Mary Jane as it would be to see him with Gwen. Straczynski, on the other hand, was born in 1954, so if he was reading Spider-Man as a kid, he would have come in before Gwen was introduced, and he would have already known other romantic interests. He was the one who wanted to bring Gwen back, actually. Quesada vetoed that idea.
I get the impression that no one really has any feel for Gwen as a character. The Gwen they want to bring back isn't the one Stan Lee wrote. It's the one that has existed in Peter's memory for the last decade or so.
The only take I've seen where I thought the writer got it right was the one in Spectacular Spider-Man, even though the usual opinion is that she's radically different because of the cosmetic differences.