At a certain point, if it's still sitting on your desk, the actual size of the monitor is so big that you find your eyes and head ping-ponging from one side of it to the other just to follow your mouse. And the distortion at the edges, just due to your extreme viewing angle, isn't helpful either. So there's sort of a useful limit to desktop monitor real estate unless you have the deepest desk in existence. The second monitor setup is still a neck-swiveling solution, but if you have all of your important stuff directly in front, and accessories/palettes off to the side, it's pretty decent. I think the situation where a 40"+ monitor is helpful is when you're mirroring on-screen content for an audience, like in an edit or flame bay.
4k and 5k monitors are availalble now, and at sizes that are still pretty useful. They're not dirt cheap, but if precision matters to you, and it should, that's probably going to work out better than a huge bargain basement screen.