Minecraft had it's giant rush, but it slowly continues to add more and more. It's ran out of a few ideas because it already included all the great things, so it's just slowly updating with new things that they can really think of
Roblox has been going on since late 06-07. I have strong faith that Roblox will continue to strive. Perhaps the community is stuff, but the engine runs nice compared to ours, and that's on it's best terms. The engine runs great if you have a decent map, if you have a ton of stuff and crap it's going to suck, equal to Blockland's.
Of course Roblox and Blockland run on different engines so comparing them would be like comparing Call of Duty to Garrysmod. Roblox has it's advantages like brick collision and movement, buildable vehicles and Lua, whereas Blockland has it's graphics, smooth lighting, weapons, and Modding / Scripting. Each game has it's unique purposes and you can't really judge one better than the other.
In terms of it dying, I haven't checked my account in a long while, but I'm sure they still make money and bring in a large number of newcomers. Blockland is just as well, and Minecraft just had a giant ass population to begin with. Everyone and their 8 year old sister plays Minecraft.
