Sacrificing more of my screen real estate for showing things I do not need is not an improvement.
Even with the left bar switched off (thanks for this option, as such) I now see 40% of my right margin occupied by things which were previously hidden away in the margin.
Here is the Stack Overflow start page as of today:
I mean ... seriously!?
Promoting navigation aids and advertising over actual content is not "responsive" at all in my book.
The left bar adds insult to injury by doing the same on the left-hand side. I can understand how menus make some things slightly less discoverable (and frankly, the menu design on this site is really not transparent, consistent, or logical) but putting trainer wheels on everyone's bikes just because there are some beginners who cannot yet ride without them is a terrible design.
I like Kevin B's proposal to move the left bar into the top bar; having the main content assaulted from both margins is a losing proposition, whereas having stuff tucked away at the top (and ideally scrollable out of sight ... though that too is now a tick box you have to discover in your preferences!) so you can get your designated window filled with the actual content you want to look at would work pretty well. If you have to make the top bar higher or more complex to accommodate proper menus, so be it.