>> "Sublime Text 2 is mostly coded in C++ and uses a custom UI toolkit." ... [0]
How many other devs/teams are going to reasonably take that on.
(And how do you think the debugging story of his custom framework compares to Chrome Devtools).
This strikes me as a sort of devolving-human standpoint that might lead to someone in 2050 asking how anybody could possibly walk 10 miles unassisted.
In the past, small teams of talented developers have written an entire game + game engine + associated tools from scratch, countless times.
Doing in-depth custom UI development for the desktop as part of creating a text editor does not bin as 'Mountain-moving; Don't even try' difficulty.