On one hand, time and effort is wasted doing the same thing 100 times with slight differences rather than just working on the same thing. But on the other hand, in my opinion these differences are very important. For example, you listed that only a few desktop environments are necessary. While that may be true for the vast majority of Linux users, the more fringe desktop environments/window managers are great. For example, I use i3wm daily and it's a pleasure to use. While some of the DEs listed in the article may have some tiling, the WMs designed specifically for tiling do it better (in my opinion). That's the glorious thing about there being many tools that do ostensibly the same thing, you can almost always find a tool that fits your niche use case.