There's an interview with Rozz Williams, who was one of the least mainstream musicians out there, where he talks about how he refused to listen to Nirvana, because "they were too popular, like Star Wars." Then when Pat Smear of The Germs joined he gave them a listen and thought they were great.
In hindsight, it's also hard to imagine a band as heavy and at times ugly-sounding as Nirvana being as popular as they were then in today's music environment. Smells Like Teen Spirit has some serious crunch, and In Utero is an abrasive masterwork.
So D&D was cool before it went lamestream like your favorite band?
But back to the subject at hand, while I cherish my old memories of playing D&D back in the day, it's easy to gloss over the bad stuff about it being more Kvlt (and debatably, it had stopped being Kvlt by the time I even discovered it, despite the Satanic Panic still being in full swing). It's easy to forget about being stuck with terrible DMs that made up stuff for sole purpose of feeling superior to their players, players that would try to kill other characters to get at the magic items they had, about never even knowing that this or that awesome product existed because there was no way to find out if it wasn't on a shelf in a store or had an ad in Dragon magazine.
It's okay to love both the past and the present. There's space for the kid in me that still re-reads the original Dragonlance trilogy, adores the atmosphere-setting black and white art of the original DMG, and the adult that loves it every time we see a tiefling knitting a scarf, cartoony art, and more inclusivity in the game.
Even if D&D feels more mainstream, it's also brought out a ton of creators that are doing their own thing and making D&D even cooler.
But there's one thing I can kvetch about with the mainstreaming of D&D, and that's how much the prices on used older products has jumped.