> I have to resize my window to actually read the text [...] we live in a world in which many people now have bigger widescreen monitors

I'm sorry, I just don't understand this whole "people have widescreen monitors, so designers need to decide how wide something should be" idea. "Because people have wider monitors than they used to" seems like a meaningless excuse to me, since windows have been resizable since Douglas Engelbart's Mother of All Demos in 1968 (more than 40 years ago).

In my opinion, when you choose a size for your web browser, you are signaling to the webpage which region of your screen you would like to have filled with content. If your screen is too wide to read full-width text comfortably, you should just keep the web browser resized to 83% of your screen width (or whatever) instead of having it maximized all the time. Most web browsers will even remember your preference from one time to the next. Plus, if you do want something to fill your screen (and you don't use Mac OS) it's one click to maximize the window temporarily.

Nothing's ever stopped designers from over-designing webpages (many people would point to scroll-to-animate webpages), but I really don't think that maximum width (or, worse, minimum width!) is some necessary thing for designers to dictate, since it's so easy for users to choose a width that's comfortable for them.

You say that but your average user isn't going to make sure they keep their browser window to some arbitrary amount. People just tend to makes it fullscreen, myself included.

One line of css to the head and it magically is a perfect website (though left aligned)

    body{
    max-width:40em;
     }

It's laid out by the browser. You should blame the browser -- not the data it tries to display -- for its bad UX.