Vlad I don't believe that to be a proactive solution — I'd even argue it's a reactive solution. There are countless bad domains and whatever used for tracking, and filter lists/ITP reactively block them. It's, however, impossible to be able to list every bad thing (badness enumeration). Sanitizing on close is a comprehensive solution that systematically solves tracking via cookies. It makes persistent tracking not possible without the use of fingerprinting, which is a whole separate problem.
Don't get me wrong, ad/tracker blocking isn't completely useless, but only if they're used as a helping hand. They shouldn't be considered a primary means of defense because they're inherently flawed.
Sanitizing on close is already a feature of both Firefox and Chromium browsers, and so I think users would expect such a feature in a browser that's "private by design."
