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I see no aplicability for scale, speed or technical feasibility here
If the Mahattan Project had use Waterfall, it would never have started, because you cannot specify that which you do not know how to do. Or even know if it is possible; eg. its technical feasibility.
You have to experiment (prototype), and feed what you learn back into the spec, and use it to choose the direction of your next level of experiment. Waterfall doesn't allow that. Waterfall is useless. Period.
Just in case, as seems likely from your rather incoherent post, you've misunderstood my post to which you replied, I was supporting the P6 cabal against the notion that they should have finished the spec before starting implementation.
And to put this firmly into the world of reality rather than speculative theory, here are a couple of unknowns from the more tentative parts of the P6 spec, that simply cannot be answered. Neither from the existing knowledge within the designers/programmers heads, nor from the existing research literature:
Whilst both those concepts are named in the Parrot/Perl6 specifications, they are, as yet, absent from the implementations, because there are no answers to those questions in the existing body of knowledge.
One can speculate on the basis of knowledge of existing similar concepts, but until someone actually tries to implement them, and use them in a real-world environment, all such speculation is just that, and nothing more.
As such, Waterfall's only answer to these parts of the requirements, would be to omit them completely from the specification.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.