I'm very skeptical of reasoning like "it was like that in ancestral environment so it must be good". There are at least three reasons that makes me uncomfortable with the reasoning :

1. Even if we consider evolution to be a perfect optimizer (which it is not), there is a huge difference between "our digestion system is optimized to make the best possible use of food X" and "food X is best possible food for our digestion system". If you made an algorithm A optimized to transmit data on a noisy channel N, it doesn't mean the algorithm wouldn't run better on a less noisy channel C. There may be an algorithm B that work better on the clear channel C than A, but still, A can work better on C than on N.

2. Evolution doesn't optimize for the same purpose we do. Evolution doesn't optimize for us to live long, it has a very low pressure to make us live past ~60, for example.

3. We have completely different lifestyles and activities than we did during paleolithic. And the optimal diet very likely depends of lifestyle and activities.

That said, what would convince me to do a diet is not a plausible-sounding reasoning, but some evidence of short-term and long-term effects on a sane sample size, with a control group. Something which seems very rare in the diet field, saddly.