It only breaks the grid
visually (and not by much); for
layout purposes, which is what you depend on, the grid is still intact.
Why do we keep using fixed-width fonts? Because a lot of stuff has been written that depends on a columnar grid. Most notably, terminals are absolutely predicated on it and fundamentally cannot support any other mode of operation.
To begin with: visual alignment, ASCII art, diagrams using box-drawing characters, &c., both in code and in the output of diverse tools:
def function_name(and_long_parameter_list,
so_that_it_wraps="like so"):
pass
ABC_D = 1 # Explanation
ABC_EFG = 2.0 # Another one
ABC_HI = 34 # A third one
┌────────┬─────────────┐
│ Tables │ Using │
┝━━━━━━━━┿━━━━━━━━━━━━━┥
│ Cell │ Box-drawing │
│ Cello │ characters │
│ Viola │ ⋮ │
│ Voilà │ │
└────────┴─────────────┘
error: cannot find macro `behold` in this scope
--> :1:13
|
1 | fn main() { behold!() }
| ^^^^^^
$ ls -la
total 43210
drwxr-xr-x 17 root root 4096 Jan 1 23:45 .
drwxr-xr-x 17 root root 4096 Jan 1 23:45 ..
lrwxrwxrwx 1 root root 7 Feb 29 2020 bin -> usr/bin
drwxr-xr-x 2 root root 0 Dec 31 23:59 boot
⋮
(I included the ⋮ in the box drawing table deliberately, because it demonstrates a weakness in the scheme: terminals and most monospacy text editors force stuff into the grid, just clipping or overflowing the cell if a glyph has to come from a fallback font, but most other things don’t, so you end up with visual alignment breaking if the fallback font used has different metrics. Also the whole East Asian Width thing and ucwidth and whatever is
super messy. Your monospace font may or may not include the box-drawing characters, but it’s much more unlikely to include ⋮.)
Terminals are also built on columnar behaviour; there are escape codes for moving the cursor to such-and-such a line and column, for example, and things like a side-by-side split require columnar behaviour.
Text editors can go non-monospaced, but it breaks various content for the reasons discussed, and you’ll need monospace for any terminal because loads of stuff will break otherwise, so combined with inertia, even editors that support proportional fonts aren’t often used that way if they default to monospace.
(Me, I’d rather like to use a proportional font while editing, but I’m not moving off Vim for it, so I’ll probably never get it. But for presentation, I like to go at least partially proportional with the monospace font Triplicate’s Poly variant, which breaks strict monospaceness, widening characters like w/m/W/M and narrowing characters like i/j/l/1. As for what Monaspace’s texture healing, I like it most of the time, but am not sold on cases like some_function_w_, where the last two underscores are markedly shorter than the first and it feels unbalanced.)