But at some point doesn't one need to think at the granularity level of clusters? In other words, there are natural limits to how big the hosting environment can grow, at which point performance and efficiency with respect to network topology and other host-system factors (in the broadest senses, e.g. at the level of data centers and the tech-pieces from which they're built) must be considered.

For reasons of convenience, in the case of some (many?) applications, perhaps those considerations can be ignored and the "container fabric" can simply be thought of as perfectly homogeneous, is that the idea? And if one grows beyond the point where that simplifying assumption doesn't hold, then it's time to switch to an architecture where clustering is explicitly addressed?