Not a physicist yet, temporarily a high school physics and maths teacher until I can start my PhD

Fe-56 is the heaviest nucleus that releases energy when other nuclei fuse into it. Everything heavier requires energy, that has to come from somewhere else, to fuse. All things tend to keep doing stuff that release energy, and they don't like to do stuff that requires energy. So, in a long enough amount of time, nuclei keep fusing together while it releases energy, and stop when it starts to require energy.

At least that's what happens inside regular old stars. The vast majority of them will have an iron core after a certain amount of time.

It pretty much only takes nuclear physics into account though, whereas the actual universe is a lot more complicated and will thus probably not turn itself into all iron.

Least Energy. I.e. yes. Does be pretty good.

Not a physicist, but I thought the heat death of the universe also involved all the matter being sucked into black holes and turned into pure energy. There's a big chunk converted up front in the accretion disk, then the rest is converted into hawking radiation as the black hole(s) evaporate over the oodles and oodles of years.

Whether or not there are also lumps of iron-56 or other matter floating around in the cold void probably depends on the real truth behind dark matter and dark energy and their long-term behavior.

This is an important question with real-life implications.