Why would you do it for every tab switch? You do it once to fit your preferred width and all websites adjust. You would probably have a shortcut for that in your window manager.
There was a great time at some point in the past where everyone went all in on responsive design and most websites behaved well, what we have nowadays is a regression in usability (at least for power users).
Some websites have one column of text. Some websites have a menu on the left as well. Some websites have menus on the left and on the right.
So no, I would not need to adjust it once as websites still have different variables that would have an effect on the line width for text.
Even with good responsive design you will run into that. Iff you are saying that with good responsive design elements move out of the way of text then you aren't really a power user in my book. Because that means you are settling for a mobile design on desktop.
In addition to all of this, I simply don't want to maintain a floating browser window as that can be a distraction. I rather have the website I am looking at and trying to focus on take up my entire screen.
There was a great time at some point in the past where everyone went all in on responsive design and most websites behaved well, what we have nowadays is a regression in usability (at least for power users).