That explanation seems to require:

1. "everything that we need" is optimal
2. What we needed in the ancestral environment is exactly the same as what we need now

Already, there are a lot of substances I ingest that cause varying effects - increased productivity, increased creativity, increased fun, decreased pain - that each has its own tradeoffs. Even if it exists, I think the 'plateau' doesn't account for everything I care about, and there is obvious room for improvement.

And it would be really weird if the plateau didn't have some little peaks and valleys on it.