XMLTABLE would be the headache. Using the standard name for something that ain't the standard function has not left any painless way that the standard function could be added. OTOH, it has only been in the wild since 10, so renaming it to something else (xpath_table?) will probably be more painless if done soon than it ever would be later.
I don't share your opinion. XMLTABLE implements subset of standard. More it is well compatible with Oracle (in this subset).
If we have library with XPath 2.0 or higher, we can continue with it.