Could be the Waterfall model the most costly mistake in the world? The Agile professional inside me says "Yes, it could be!". Despite this, I do agree the people belief "Choose the vehicle which fits your culture!".
I adopt the above because both environments have benefits and pitfalls. Anyway, it is extremely simpler to switch from Waterfall to Agile when needed and it is not vice-versa. This because Agile is about core values, extremely different in these two environments. Check this out: Agile Manifesto! The first value is enough to see the main difference.
If you agree these and want to learn more on these topics, please read this article.
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