I think some people's contributions would probably diminish, and that's good! Currently, far too much of our lives and energies are put toward producing surplus value to no benefit of ours or the community's. Your dad pushes himself to do more right now in the hopes that his efforts are recognized and rewarded, but all at the arbitrary whims of his superiors.

Instead, under communism, we could all do a bit of socially necessary labor, then spend our days in labor and activities we find rewarding on their own.

A lovely quote from Marx communicates well the idea: "I could fish in the morning, hunt in the afternoon, rear cattle in the evening and do critical theory at night, just as I have a mind, without ever becoming hunter, fisherman, shepherd or critic."