My GPX tracks¹ start with
<?xml version="1.0"?>
<gpx xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" version="1.1" creator="http://www.delorme.com" xmlns="http://www.topografix.com/GPX/1/1">
<trk>
and end with </trk>
with no </gpx>
anywhere.
I am pretty sure there are no practical drawbacks to this, but what might be the benefit of "fixing" it in thousands of files?
¹Several years of hiking and biking.