This can be debated. Although Rust/H1Z1/PUBG were certainly popular battle royale games before Fortnite, Fortnite was the only one that brought it mainstream, outside the hardcore gamer ecosystem.
Fortnite became a place for people to hang out: https://twitter.com/anoopr/status/1071921080485457920
My brother, a college football player, plays it. My sister, an Instagram addict and high schooler, plays it on her phone while shopping at, I dunno, Forever 21 I guess.
This is new, and worth paying attention to.
GTA V is the top grossing game of all time and sold 90 million copies on 3 platforms. Minecraft has sold 150 across way more platforms and has been around way longer.
This isn't about video games anymore, it's about tapping into something bigger. Even Minecraft couldn't crack into the market segments Fortnite has.
I see that minimaxir has been downvoted so it seems "mainstream" has different meanings.
I don't play video games and don't keep up with them. I've never heard of PUBG. But nevertheless, I did hear about Fortnite. Why? Because a business analyst on Youtube did a business case study[0] about Epic Games and the development history of Fortnite. That was unusual because he typically analyzes non-game companies[1] like Amazon, Uber, Stripe, GoPro, Theranos, etc.
My point is that there must have been a noticeable step change in popularity level between PUBG and Fortnite such that it entered his consciousness and justified a business case study from him -- which in turn entered my consciousness. If "mainstream" isn't the right word to use to describe Fortnite's greater reach to non-gamers, I don't know what a substitute word would be.
Also, I know what the "floss dance" is and that it's in Fortnite. If PUBG or other video games have named dances, I don't even know of their existence.
Now, I do agree with you, but more to do with your point about the flossy dance, since seeing stuff from games leak into real life consistently is pretty mainstream.