a) People don't know their own needs until you show them what their options are. Are you familiar with the literature on optimal line length that is quoted in the OP? If so, good job. I think it's fair to say 99% of people aren't. Consequently, if you asked people if they wanted an optimal reading experience, then they would have to concede that they are not knowledgeable about what that means, and thus that they should adopt different settings than what they would instinctively pick.
b) The alternative is not that people are "idiots who don't know how to use their window manager." I don't want to have to use my window manager. Manually resizing windows for each tab to get the right line length is a huge waste of time and seems idiotic to me. I want a professional designer to make it so I have the best possible reading experience on the websites I'm browsing. Because I'm humble enough to know I couldn't reach that standard by "using my window manager".
That argument completely falls flat if you consider the actual result: https://i.imgur.com/7mAQy5H.png It enrages me to no end that this is sold as an improvement.
• Who wants to read four to five words per line? THIS LINE SIZE IS ACTUALLY UTTER ASS.
• In the good design, it was easy to resize to window to make the line length narrower. However, in the bad design, there is no recourse to make the line length longer.
The "actual result" looks nothing like that in my browser, so I can't really judge, although I agree that it looks terrible on your screenshot (text overlapping in links, etc.).
However, the line length, while suboptimal there, is pretty commonly encountered — pick up a newspaper and look how many words fit in a column.
b) The alternative is not that people are "idiots who don't know how to use their window manager." I don't want to have to use my window manager. Manually resizing windows for each tab to get the right line length is a huge waste of time and seems idiotic to me. I want a professional designer to make it so I have the best possible reading experience on the websites I'm browsing. Because I'm humble enough to know I couldn't reach that standard by "using my window manager".