Excellent looking customized theme.
Although the question is a valid one, it does have its disadvantages, as pointed out. For example, with any upgrades to core files, etc.
However, from a practical point of view, the "name" next to the user icon already has that feature as pointed out as is part of the OSSN files as default, no change required, except perhaps by a colour scheme which can be done with STYLER component and the colour scheme hardcoded in STYLER if required.
As to the actual question of making the User icon clickable, the biggest drawback to that is that these icons may not necessarily be unique to users. USERNAMES are unique, so if the site has DISPLAY USERNAMES active then you will get unique usernames that can...maybe...have several users with the same icon... If I use an image that is freely available on the internet and someone else does the same with the same image, but we both have different usernames, then I can see that may cause confusion.....
It is just a thought as we use USERNAMES for that very reason, as it is unique and the name can be clicked on and not the icon, so you must consciously click on the name, which is unique, to get to the profile.
Just our thoughts, as we had thought of this a while back but dismissed it.
Cool theme never-the-less.