Greyhawk is more alive than Darksun, Spelljammer (which really isn't a "world" perse) or Dragon Lance as WotC is still releasing explicit Greyhawk content in the form of modules. However, I'd argue that that latter three are dead, and Greyhawk is dying. Each has its own flare that makes for interesting content. The love of Dragon Lance comes primarily from the novels and not the RPGs. If the novels are not selling then it's dead. For Greyhawk, Darksun and Spelljammer the opposite is true. They have fans because people played D&D in those settings. The love of Greyhawk is classic swords and sorcery that the earliest years of D&D were built on, considering the game's creators built and played in that world. There's still a place for each of these but whether or not WotC decides to keep them alive remains to be seen.
I still run Greyhawk and Spelljammer for 5e. I've wanted to play in and/or run a Darksun game but it never gained ground. I loved what they did for Darksun 4e but we never got around to playing it. For a decade I've tried and had three different Greyhawk campaigns with different players. I suspect it's because swords and sorcery the setting is far more open since anyone can picture themselves in a world with a little more magic and a few more monsters, rather than a weird outerspace adventure, a world teeming with dragons, or a dead world ruled by psionics and dracoliches. The niche aspect of these other settings makes them intriguing, but also limits them.
Insofar as Ravinica as a new and alive setting, I'd put it in the same realm as Spelljammer or Darksun - it's a niche world of gaming. I know lots of DMs and groups that use the WotC supplements for Ravinica, but none who play the setting. Mind you, I'm currently playing MtG with the D&D expansion every week and not a single card gamer is interested in the RPG. One of the players who holds tournaments has a homebrew world that borrows quite a bit from Eberron. As he told me, the Guilds for Ravinica work just fine for a card game as they separate the decks on fundamental differences in magic but that doesn't work well for RPGs as the parties and their interests vary greatly. The monolithic nature of the guilds limits the game, as is designed, but it also limits the DMs. Eberron's guilds were designed in an RPG, and their economy is built in the system you play, not a different system and then imported. Ravinca has great ideas you can steal, but no one is using it as a campaign setting. Dragon Lance has similar problems with their Knights and uber NPCs. It's great for novels but not for RPGs.
WotC really has the power to kill or revive these worlds. As we've seen with movies and television, sometimes the worst thing you can do is try to revive something that you thought was dead.
