Q: Would it be possible in the distant future to directly convert matter into energy?
In order to convert matter into energy requires us to get past a few conservation laws. Make a black hole, feed it any kind of matter, and collect the energy it generates. Quick aside: This stuff here about “feedability” and whether or not particles can fit into a black hole are non-issues. So, maybe, in the far off future we could (somehow) create thousands of tiny black holes and harness them for power. The danger is that civilization may forget to move their black hole power sources far away before they burn out.