Extension element children

Is there any reason why the extension * has * to be at the end of
the document? The reason I ask is that there may, if so, be an issue
with further extending the document with minor versions within UBL,
mightn't there? Such extensions would * have * to be at the end of
the document - for backwards compatibility. They would therefore
replace the extension element as the last element in the document.
I'm hoping the extension element is only placed at the end of the
document for aesthetic or common practise reasons.

All the best

Steve