This is clearly starting to become a divisive topic so I don't blame Steven if he wants to play it safe.
I do have to say though, that personally I fall heavily in favour of of extending the header bar to over the left panel. I use MB by keeping several tabs open at all times. Some of these have the left panel collapsed, and some expanded, depending on the purpose of that tab. I have always found it to be one of the strangest and most frustrating quirks of MB that as I switch between tabs, the back/forward buttons jump left and right. It just isn't logical and it certainly isn't intuitive. I still find this catching me out (with my mouse wandering to the incorrect location) even after years of daily MB use.
The feedback on this particular part of the proposed changes DID start positively and, to be honest, I do not understand how the loss of a few mm of vertical space for the left panel could even be considered an issue.
Doing things the way other programs do it may not be an argument in and of itself, but surely it is indicative that many others have thought through similar design problems and come out with the same solution. UI elements/buttons that do not remain in a consistent location simply make no sense. It is a shame that this thread has descended into so much negativity and resistance to change. Of course not every solution will work for every person, Steven has repeatedly stated that these options would be just that, options. This flexibility is something he has somehow managed to keep with almost every reworking of key aspects of the program in the past. Yes, it has led to a large amount of options that may not be apparent out of the box, but that core usability has always remained, all the while adding more and more excellent features for us hardcore users and settings tweakers. All of these changes have been guided by a forum that fostered and encouraged change with helpful and thoughtful feedback. Lets not lose that.