Well exclude-result-prefixes does not change the namespace of elements in your XML input document.
What is it that you want to achieve?
If you want to process elements in the namespace urn:schemas-microsoft-com:office:spreadsheet like that Workbook element and want to strip the namespace then you need to write a template for that e.g.
Code:
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="2.0"
xmlns:mss="urn:schemas-microsoft-com:office:spreadsheet">
<xsl:template match="mss:*">
<xsl:element name="{local-name()}">
<xsl:apply-templates/>
</xsl:element>
</xsl:template>
...
But I am guessing what you might want to achieve with the exclude-result-prefixes, it is better you explain what you want to transform your Workbook element to.