The theory of relativity overturned the concept of motion from Newton's day, by positing that all motion is relative. Time was no longer uniform and absolute. Physics could no longer be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved spacetime. Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds.

https://en.wikipedia.org/wiki/Theory_of_relativity

The problem is, there are losses in the conversion process, and there are limits to the efficiency of the conversion of thermal energy to other forms of energy, as described by the second law of thermodynamics. Energy needs a device to transform it from one form to another (like the plant that uses photosynthesis) so this also limits its ability to convert.

The other problem is that stars are constantly expending energy, and at some point will have released all their energy. Now, think about what happens in the winter when the sun's rays are merely less direct away from the equator. It
gets cold, obviously. Now think of what would happen if the energy from the sun completely disappeared. The residual heat would all eventually dissipate into space as useless thermal energy, never to be recovered in a useful form again.

So the eventual end (barring intervention by God) would be that the useful energy of the stars will be completely expended and converted to another form of energy. It eventually would all be converted to thermal energy and dissipated off into space as the energy from the stars ceased, or would be locked up in matter with no device or mechanism to convert it to another form as all matter approached absolute zero. Result? No more useful energy, but yet the same quantity of energy in the universe.

https://en.wikipedia.org/wiki/Effort