Martynas,
I think you misinterpret the thread. There is no discussion not to build on the datatypes defined in http://www.w3.org/TR/xmlschema-2/
What we are doing is discussing compositions of elements, all typed to xml datatypes, that shall be able to express scientific and engineering requirements as to statistics, signficant digits (except perhaps for duration, none of the data types in http://www.w3.org/TR/xmlschema-2/ supports that), as well as means to express uncertainty and confidence intervals.
Many existing xml schemata define such compositions, all squarely built on http://www.w3.org/TR/xmlschema-2/ - wikidata is certainly not unique in this effort. If you can point the team to further well reviewed solutions, this would be very useful.
Gregor