Evolution can be stuck in local maxima determined by some path it went down a long time ago. If life happened to evolve an inherently inefficient metabolism, it would be stuck with it forever because you can't really go back on something that fundamental (we share glycolysis mechanism with bacteria).

And, even if evolution automatically did win, plants did not evolve to feed us. They evolved to reproduce themselves, and we have incidentally hijacked them to use as food sources.

But they did evolve to extract useful energy from solar radiation. Assuming it would be easy to tweak photosynthesis is ridiculous.

Evoltution is great at tweaking, and would be hard to beat, but it might be possible to build something completely different. It would also be hard to tweak a bird's wings, but we have built aircraft with completely different designs that fly many times faster than any bird.