"Quick Launch" link is missing in the "Look and Feel" section of SharePoint site settings

As per one of my customization task i was adding various links in the Left Navigation of my site by using the "Quick Launch" option under the "Look and Feel" section of the site settings.

The issue was, suddenly the "Quick Launch" link was missing from the site settings page. I tried various options and couldn't find the exact reason for the problem. I completed my work by accessing the following URL as a workaround.
http://mysharepointserver:6666/sites/myplayground/_layouts/quiklnch.aspx

I was investigating about this problem and found the following points as root cause of the above issue.

1. "Quick Launch" link will be replaced by the "Navigation" link if we activate the "Office SharePoint Server Publishing Infrastructure" feature. (This was activated by another user simultaneously accessed my site. That caused the confusion)
2. We can still do all the functionalities provided by "Quick Launch" using the "Navigation" option with added features like audience and ordering of the links.