I'm not sure what you mean by this sentence. Are you saying it would be dangerous if writing a new engine in 2020 were impossible (agree) or are you saying that making such a statement is dangerous (hard disagree).
Making a new engine in 2020 is hard. Making a new engine that becomes a sustainable, maintainable project in the medium term MAY BE impossible (Flow looks amazing but I imagine funding and maintaining it long-term will be an immense challenge). Discussing this fact, and the reasons that it is so challenging is important if we hope to avoid the dangers of a monoculture.