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I think these are assumptions, rather than fallacies, and there are
environments in which they hold.
Schemas are often used to create a contract between the producer and
consumer of data. You promise me that the information you send me will
fulfill the criteria expressed in the schema, and I have the right to
reject data that does not meet these criteria. That's a common and
legitimate use of schemas.
But the schema expressed in the XML schema language is often not the
entire contract; there may be several contracts, expressed in different
languages, and some of these may be constraints of the application
domain. That's why schema extensibility is so important.
Of course, as a consumer of data, I may also impose conditions that I
don't tell the producer about, or the producer may impose conditions
that I am not aware of. I may also choose to look at data even if it
does not fulfil our contract. But that's not what's in the contract.