At one, time, I was intrigued by the possibilities of ios handoff, which promised me the option of starting my work on one computer and resuming it on another. (It's not compatible with my iPad 3 and it wasn't compatible with my old 2009 iMac).
Had I a pair of more recent devices, I could have tried it out and decided whether it fit my style. Alas no.
Apple periodically decides to exploit newer technologies and take them in a whole new direction. A ten year old mac is likely to have many such missed opportunities.
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Hence why ~40" is roughly the magic number. It gives you that big screen next-gen vibe without being cumbersome. Ideally a curved display would be the best at that size tho because it eliminates any color shifting. A very important aspect about 40" is that it's a standard TV size. And TVs are what are pushing the 4k revolution. Right now it's possible to get 40" 4K 60hz 4:4:4 TVs for about $600. When that price drops by about half they'll flood the PC market and compete directly with regular monitors. You can already see on slickdeals that people are interested in using 4K TVs as monitors.
Figuring out what's ideal sometimes requires a great deal of money and time.
I can justify my 27" 5K + 20 inch 1080p setup as "works for me." Is it ideal? Far from it. The 1080p screen isn't IPS, and I just cannot get it to work on windows 10, as well as it does in Mac OSX.
Meanwhile, the 5K aspect of the main screen is so very easy on the eyes for looking at text-- and having lots of text is presumably why you'd want huge desktop in the first place.
If I had a 40 inch 4K screen for looking at text, it would be harder to read, but I could store more stuff in my peripheral vision. Honestly, I'd have to spend the money and test it out-- and I don't have that kind of money for such experiments. If I had millions of dollars, I could build more interesting prototypes like 8K displays and what not and land on the perfect configuration for what I do. And it probably would not be a pair of 27 inch 5K displays, or a 40 inch 4K display, but something that hasn't quite been invented yet.