Maxperson
Morkus from Orkus
Only if a TPK is the only fail state, which it isn't. You can end up running away from the fight as a fail state. In a game where coming back from the dead is rare, 1 PC death is a failure. The enemy could rob the party and leave it unconscious. The party could be taken prisoner. And more.That would mean on average a game of D&D would end in a TPK after 3 fights (2/3 win ratio with full refresh)
All a 2/3 success rate means is that on average, out of every 3 fights the party will have bad stuff happen once.