The OP says that double-spacing is an obsolete holdover from the typewriter era, where the extra space made monospaced type easier to read. I'll go one further and say that spacing sentences using space characters -- any number of them -- is obsolete.
In the present era, the act of typing is separate from the act of typesetting. The comment I'm typing now may be typeset in Arial in Chrome, typeset in Ubuntu Mono in Emacs, or read aloud by a software program to a blind person. No prescribed number of space characters is going to be appropriate for all cases.
The reading software, not I, should be responsible for locating my sentence breaks and setting appropriate spacing there. Perhaps in the future we'll assist the software by marking up sentence breaks using a special character sequence. Ironically, a double-space would serve that function pretty well.
In the present era, the act of typing is separate from the act of typesetting. The comment I'm typing now may be typeset in Arial in Chrome, typeset in Ubuntu Mono in Emacs, or read aloud by a software program to a blind person. No prescribed number of space characters is going to be appropriate for all cases.
The reading software, not I, should be responsible for locating my sentence breaks and setting appropriate spacing there. Perhaps in the future we'll assist the software by marking up sentence breaks using a special character sequence. Ironically, a double-space would serve that function pretty well.