Alignment is the problem here, not the proportional font.

I gave up alignment in my code many years ago, back when I was still using monospaced fonts. I gave it up because I was really tired of fiddling with spaces when I changed a name in code like this:

  myType someFunctionName(someType      someVariable,
                          someOtherType someOtherVariable,
                          int           foo) {
      ...
  }

So I started doing this instead:

  myType someFunctionName(
      someType someVariable,
      someOtherType someOtherVariable,
      int foo
  ) {
      ...
  }

I found that I just didn't care that "someVariable" and "someOtherVariable" weren't lined up one above the other. In fact, I actually liked the fact that the variable names and types were closer together, so my eyes didn't have to keep scanning across a sea of whitespace to match things up. And I really liked not having to add and remove spaces every time I changed a name.

As a bonus, the overall width of my code was often much less, as in this example.

Later I discovered that using this style, my code looked the same in a proportional font as it did in a monospaced font. So I switched to a proportional font and never looked back. But the code is still just as readable in a monospaced font.