In the preface to Landau and Lifshitz's Statistical Physics, they comment the following
In the discussion of the foundations of classical statistical physics, we consider from the start the statistical distribution for small parts (subsystems) of systems, not for entire closed systems. This is in accordance with the fundamental problems and aims of physical statistics, and allows a complete avoidance of the problem of the ergodic and similar hypotheses, which in fact is not important as regards these aims.
After explaining that they will consider (macroscopical) subsystems of the whole closed system, they claim
A fundamental feature of this approach is the fact that, because of the extreme complexity of the external interactions with the other parts of the system, during a sufficiently long time the subsystem considered will be many times in every possible state.
But this is very similar to the ergodic hypothesis, is it not? From my Stat Physics lecture notes,
Given a sufficiently large time, any closed system will approach arbitrarily closely any point in phase space
I don't understand how considering subsystems overcomes the need for this hypothesis, neither what advantage it has. It seems to me that the authors are using the complexity of the interactions between the subsystem and the rest of the system to grant credibility to the claim that the subsystem will eventually have populated the entire phase space. But why isn't this equally credible to stating the ergodic hypothesis?