I think it's even worse: the big problem with elastic tabstops is that you can't just adopt them on your own. You are requiring everyone who reads your code to use them if they want to see it formatted properly.
Proportional fonts are different. Anyone can use them without interfering with readability regardless of the fonts and editors that others use. If you look at my code, you will never know whether I wrote it in a proportional font or a monospaced font.
> You are requiring everyone who reads your code to use [elastic tabstops] if they want to see it formatted properly.
It is not quite as bad as you make it sound.
While editors which haven't yet implemented the elastic tabstops mechanism may not align some text properly in files where tabs were used with elastic tabstops, the problem isn't that bad. All leading tabs (indentation) will be okay, and the chances of text not aligning correctly diminishes as the width between tabstops increases.
Proportional fonts are different. Anyone can use them without interfering with readability regardless of the fonts and editors that others use. If you look at my code, you will never know whether I wrote it in a proportional font or a monospaced font.