It's not like this is limited to just a few pairs of letters like 'mi', or just pairs - if you've seen Fontemon https://www.coderelay.io/fontemon.html you know the sky is the limit for what the rewrites can do!
The same letterform having different widths just looks bad and wrong.
It's not like kerning, which always looks good when done well (for proportional fonts).
In "gimme" you could have a rule that when any character is next to itself both occurrences must have the same width. Wouldn't that fix the "gimme" example?
Everything else would just look like a proportional font.
Maybe it would influence coders too much and subconsciously make them choose names that line up well though...
1) Horizontal spacing needs to align vertically between glyphs of the same letters on separate lines. This is the example I give elsewhere of a letter "i" being in a very slightly different position than the same letter just two lines above it. I personally find this extremely disturbing and unpleasant to look at in monospace.
2) The glyph of a letter needs to be the exact same between instances of the same letter that are in any kind of visual proximity. This is the "gimme" example the parent of your comment gives. AnyThinG ElSe LoOkS lIkE jumblycase. Which I think we can all agree is just plain horrible.
Interestingly enough, the fact that letterforms change as I type doesn't bother me the way I thought it would -- turns out I'm already kinda used to it from ligatures.
So points for being a clever idea to try out, but unfortunately what it does to a word like "gimme" just makes it a non-starter. Let's face it -- lowercase m's in monospace will always be ugly and cramped, but at least they're consistently so.