TL;DR
please, never ever use sed for this task !
Everytime you use sed for html or xml, you kill a kitty

(a proper XML parser) and his friend xpath, like this:
xmlstarlet ed \
           -L \
           -N w="http://schemas.openxmlformats.org/wordprocessingml/2006/main" \
           -d '//w:rPr' file.xml
A bit of explanations :
- -Ledit the file on the fly like- sed -i
- -Nset the XML namespace, if needed
- -dremove nodes matching- xpathexpression
Check xmlstarlet edit --help
Pure XQuery solution:
$ cat XQuery
declare namespace w = "http://schemas.openxmlformats.org/wordprocessingml/2006/main";    
copy $input := doc("/dev/stdin")
modify delete node $input//w:rPr
return $input
$ basex XQuery < file.xml
Using XQuery and xidel:
With limited XQuery capabilities.
xidel --xml --xquery '
    declare namespace w = "http://schemas.openxmlformats.org/wordprocessingml/2006/main";
    x:replace-nodes(//w:rPr, ())
' file.xml 
theory :
According to the compiling theory, XML/HTML can't be parsed using regex based on finite state machine. Due to hierarchical construction of XML/HTML you need to use a pushdown automaton and manipulate LALR grammar using tool like YACC.
realLife©®™ everyday tool in a shell :
You can use one of the following :
- xmllint often installed by default with libxml2, xpath1
- xmlstarlet can edit, select, transform... Not installed by default, XPath1
- xpath installed via Perl's module XML::XPath, XPath1
- BaseX not installed by default, package basex, full XQuery 3.1
- xidel XPath3, partial XQuery 3 (no Update)
- saxon-lint my own project, wrapper over @Michael Kay's Saxon-HE Java library, XPath3
or you can use high level languages and proper libs, I think of :
python's lxml (from lxml import etree)
perl's XML::LibXML, XML::XPath, XML::Twig::XPath, HTML::TreeBuilder::XPath
ruby nokogiri, check this example
php DOMXpath, check this example
Check: Using regular expressions with HTML tags
