Is this game really dead?

It's really not. People who say all the adults / mature people are quitting may feel like that because they're one of them - but it's a cycle. Like a life, for example. New people are born, and they get older. As time goes on, more are born, and more die off. Minecraft's playercount is not decreasing, but servers are getting worse - The game itself is alive and kicking but it's the community that's starting to get worse - not dying, mind you, but worse.