To a single plant, alone in a pot, the sun's energy is unlimited. But to a plant in the rainforest, it has to compete with every other plant for a patch of cross-sectional area with access to the sun's rays. So the energy is still scarce, and you still have a commodity (i.e. the available cross-sectional area, or land, where the sun hits). In the long term, humans will still have to compete for the sun's energy, and could still monetize this competition.

From an economics perspective, I don't really see how solar energy puts us in a different place than our current one. It's not really that different from feudalism, where we got energy from the sun via the crops on the land held by the lords.