Some people have pointed out that opening the core doesn't necessarily have to mean the end of spiral knights, we have no idea what is inside the core and a simple plot twist could solve that problem, It doesn't even have to be too elaborate since we're in a fictional world, Almost anything would work.
Besides there is some npc and I believe that it was vaelyn that said it (not too sure though) that the core seemed to be more like a cage to keep something locked in rather than protecting it, it is dead obvious that they're suggesting that what is inside the core is an evil and very powerful creature and something or someone locked it in there.
I would love it if opening the core is part of the supposedly upcoming arcade redux, it is something the players have been dreaming about since the beginning of this game.
theres no tier 4
shadow lairs are not in it, they are simply corrupted parts of the clockworks
if they ever created tier 4, it would be inside the core, and opening it would be the biggest mistake of the game (im not going into detail, its easy to figure out through common sense)
and as soon as I see someone that thinks any part of the game is tier 4 I lose all respect for them and I will be ready to correct ignorance and lack of common sense
edit: aviri, read though this and state exactly where I say "opening the core ends the game"