Should be invulnerable to damage and CC while in spawn if out of combat for x seconds, in my opinion. If you're being killed on spawn then the game is likely over, though I've had many games where the enemy would just keep spawn killing and pulling players out of spawn(Blitz) and killing them, not ending the game. Minions will eventually push and win, though the situation I described can last for 3-5+ minutes sometimes.
Anyway, the Obelisk is quite strong, but if it were to instant kill then there would be no way to escape if forced into spawn.