While filler doesn't seem to be going anywhere, I do feel like family game or casual game seems to be used as an alternative to gateway game. I don't think gateway game is going away but it doesn't seem to be as prominent as it used to be.
I think this is because there's been a shift in the board game audience, at least here in the US. (Meanwhile, over on the other side of the Atlantic, all this board game love is old hat)
Ten years ago in this country, I think there was more of a sense that games like Carcassonne or Catan or Ticket to Ride were steps towards getting into the hobby. A rite of passage, almost. However, as games like that have gotten more and more mainstream acceptance, games like those have become their own endpoint.
Mind you, this is clearly an evolution and expansion of the hobby, not the industry or culture on a whole. After all, there have been companies that have been targeting this market for a long time, like Out of the Box or Gamewright or, you know Hasbro or Mattel :P
I honestly think that part of what has happened is that the folks who discovered this broader world of boardgames ten, fifteen, twenty years ago are starting to have families of their own, kids are old enough to play these games. The hobby is creating this increasingly broader part of itself.
Heck, I just think about who was attending the first GenCon I ever went to and how many more kids and families I saw at the last one. The hobby is definitely changing, which is good. Extinction is what happens if you don't evolve. Unless you're a beetle. Beetles got it worked out millennia ago.
Now, if only there was another word for filler.