It will most likely go down like this:
Toribash gets popular
Toribash gets boring
most people leave
only a few cool people stay
And repeat.
I think when people ask when the game is going to die, they see it as a bad thing. I think it's important to appreciate how long this game has actually survived, especially considering how nice it is. There aren't many turn-based fighting games with a steep learning curve out there that are of this variety when you think about it.
Either way, I think it's lifespan from here on out depends on the 4.0 update and how much it brings to the table. 3.0 gave the game a boost due to the shaders being implemented (if I remember rightly) and its switch to a free-to-play game. I'd say that was good for Toribash's lifespan, and definitely prolonged its death, but I wouldn't say it's out of the water yet. It's slowly declining, by what I've been told (this thread in particular, I haven't played much so I couldn't say first hand) and whilst that's sad to see, I think it's gotta happen sooner or later, and when it does you should all be happy you got to ride the wave whilst you could