I get your point. It is very appealing. I do note that HTML is a definitive example where the strict version lost. Hard.
My argument here would be to do your best to not create ambiguity in what can be fed to you. The currency one is a great example. Never ever make something that takes in untagged integers as a value. It could mean too many things and you would have no method of knowing what was intended. But, once you start accepting strings, "$4" or "4 dollars", or "Four dollars" should all be on the table. (Though, yes, in this case probably best to stick with "4 USD" and complain about "dollars" being ambiguous without another qualifier.)
My argument here would be to do your best to not create ambiguity in what can be fed to you. The currency one is a great example. Never ever make something that takes in untagged integers as a value. It could mean too many things and you would have no method of knowing what was intended. But, once you start accepting strings, "$4" or "4 dollars", or "Four dollars" should all be on the table. (Though, yes, in this case probably best to stick with "4 USD" and complain about "dollars" being ambiguous without another qualifier.)