If the claim being made is "well obviously they should have just done foo and it would have worked," there's reason to believe that foo gives you a significantly less competitive product, and the speaker is not giving any evidence for why it's possible to do foo and still succeed in the market, the only thing we have to go on is the speaker's experience in that market.

A rigorous argument for why foo can be done and doesn't kill your product would be fine too, but this email doesn't even attempt to do that. What can be asserted without proof can be dismissed without proof.