But a matching template would never looked for that
element, as the default template rule for elements would be
redefined to do nothing, and the root element would match
it.

So the entire tree walk would stop at the... root element:

<xsl:template match="*"/>

    <xsl:template match="non-root-element">
      Can never be reached.
    </xsl:template>

Ah, in that way. Yes, you are very right ;)
I *always* start my new template with something like:

<xsl:template match="/" name="main" >
   <xsl:apply-templates select="specify/starting/point/here" />
</xsl:template>