[jbay808]

> For example, Many Worlds theory (from my admittedly limited understanding) says that the wave function's eigenstates each become the new reality in different universes

That's perhaps the Narcissistic Many Worlds interpretation. Another way of thinking about Many Worlds is that there's only one universe, in which: after measuring a particle in a superposition of two states, you are now in a superposition of two states. Each of the superimposed you states thinks your instrument measured a single clear result.

The wavefunction of the universe goes on propagating as usual, including both of the superimposed yous, but within each of the superimposed states, that you thinks that it has witnessed wavefunction collapse.

[ravi-delia]

It annoys me that you really have to dig into Many Worlds for a while before someone says outright what you're saying here. I guess it's because to someone already familiar with QM, decoherence is more about explaining why the newly different portions of the wavefunction don't interact than that there's a superposition at all, but to the layman the takeaway should really be exactly what you wrote.