I'm completely convinced that it's the majority, and not just the majority of gamers, but the "big" majority of fantasy entertainment overall. It was a driving force behind the impetus of writers like Howard, Morris and Tolkien. It's a huge factor in most fantasy novels today. It was talked about incessantly in the documentaries to the Lord of the Rings movies. Guys like China Mieville and the New Weird were specifically reacting against the nostalgia of the fantasy genre, which he has described as ubiquitous.
Sure, I can't assign motives for any individuals without risk of being blatantly wrong, unless I know the individual in question reasonably well. But I'm pretty confident that the concept of nostalgia is hand in hand with fandom of fantasy in general and has been from before even the roots of the genre were laid.
And yes, all fantasy games have an undercurrent of nostalgia for the Golden Age of swashbuckling adventure, even as consumers of them know that they're being sold a romanticized bill of goods. I personally believe, also, that nostalgia plays a very significant role in the OSR as well. James Maliszewski's blog was dripping with it, the way he made it almost a ritual sacrament to try and recreate a Gygaxian mileu, and sample anything and everything that he felt might have been an input of any kind into early D&Diana. Matt Finch's Old School Primer does the same.
Does this mean that every single OSR gamer is so because of nostalgia? No, of course not. Does it mean that nostalgia is a strong force in the very existence, not to mention the propagation of the OSR? You certainly haven't convinced me that it isn't. I can rather easily line up hundreds of counter examples of blogs, posts, and the two mentioned already that freely reference nostalgic sentiment on a regular basis.
Also, your hair-splitting of romanticized wish-fulfillment or escapism as not nostalgia is hair-splitting that the definition and general usage of the word doesn't support.