>If you wait a few million years I bet it gets used again several times.

You can't talk the Second Law of Thermodynamics away so lightly. There are irreversible processes, there is no way around it.

Usable energy continuously gets converted into energy that can no longer be used.

This may all seem very abstract, so here are some specific examples from Astronomy.

Suns radiate energy. This energy goes into space, becoming ever more diffuse; it can't again be concentrated in a star again.

To produce this energy, hydrogen is converted into helium. Eventually, Helium gets converted into metals (heavier elements, like carbon, oxygen, etc.). There is no way helium or metals get converted back into hydrogen: for example, to convert hydrogen into helium, energy is produced (and eventually, this energy is radiated into space); so obviously, to convert helium back into hydrogen requires an energy input.

(Note: The usage of the word "metal" in Astronomy differs from Chemistry: anything heavier than hydrogen or helium is classified as a "metal". In stellar evolution, chemical properties are usually not relevant, due to the high temperatures.)

When a star stops producing energy, there is a remainder that doesn't go back into interstellar space. For anybody whoe doesn't know what I am talking about, do some reading about white dwarves; neutron stars; and black holes.

And the part of a supernova that does get blasted off into space contains more metals and less hydrogen and helium than the original substance that made up the star. In summary, the Universe's fuel reservoirs get used up; the processes that keep stars shining can't last forever.