The last nodes are never observed by anyone, but they descend from the same physics, the same F(physics), that have previously been pinned down, or so I assume. You can thus meaningfully talk about them for the same reason you can meaningfully talk about a spaceship going over the cosmological horizon. What we're trying to avoid is SNEEZE_VARs or lower qualia where there's no way that the hypothesis-making agent could ever have observed, inducted, and pinned down the causal mechanism - where there's no way a correspondence between map and territory could possibly be maintained.