I would suggest that the basic premise of this question is flawed.
[…] I need to view who is online […]
This is not a "need" in the sense that it is required for proper use of the site. In fact i would go so far as to suggest that if this is a "felt need" you are probably doing something wrong. Stack Exchange is not intended to play the role of a social network. You don't come here to interact with specific users, you come here for the content.
There are times when knowing how long it's been since a user was active might help make a decision on how to handle their content.
Knowing when a specific individual was last seen can occasionally make a difference in knowing how to handle their content. Have they dropped off the map? Are they going to update that post that needs fixing? Have they had a chance to see comments that can now be removed as obsolete?
Chat rooms also provide functionality to see who is in the room and who is recently active. Knowing this information is relevant to the function of those tools. For the main site, a sorted list of active users is simply not relevant.
Some information about the most active users is relevant to the reputation leagues.
Adding such a feature would prove to be a distraction, subverting the way things work best.