Principle of least surprise – as a browser user I expect that the current URL is that of the page, not the part of the page I’m currently reading.
When I started using Discourse I frequently got caught out by this issue @charleswalter described. Now I know to trim off the comment number at the end of the URL when sharing a discussion. New users frequently don’t though.
Personally l think the URL in the browser should link to the beginning of the conversation. Sharing a link to an individual comment would still be possible using the link button.