Just for the record on this nice and historic question, I will go ahead and say that:
In the (today extremely unusual) circumstance that you are using a monospaced fount, I'd suggest really it is probably better to continue with the two space rule.
Recalling that, the very reason for the two-space approach is/was simply for clarity:
Examples could include:
(1) You're an art director, and you're making a poster that features a sentence fragment in "old-fashioned broken typewriter look" (so, it's a "crime novel look"). In this case you should do it because, well, that's how typewritten pages look.
(2) In the very unusual circumstance of writing computer code. The comments (ie, English human-language sentences in the code) are, indeed, in monospaced fount. FWIW I'd point out that, thus, it's better to use the double-space in such writing. (Indeed, this is a good example where clarity is everything.) for example FWIW I do this and all employees have to. It would be difficult to say if this is "common with programmers over 30" but anyone who's used to typing monospaced, this would be the only (I can think of) present situation where it still happens.
You could click to say github to find swathes of example computer code in various computer languages.
(An interesting aside there is that conventions like capitals appear to have fallen by the wayside, but the double-space remains a good sentence clarifier.)
Note - perhaps someone else can think of another example where, today, one actually writes in monospace. Even text-messages, say, are now flawlessly typography'ed.
(3) If you are - say - a spy, or in general, for some reason you are writing a message on a monospaced system (so, you're a spy sneaking a message out using an old typewriter), I encourage you to use the double-spacing, again simply for clarity and decisiveness - you don't want a comma even possibly confused with a point. (I guess in old-fashioned telegrams using "STOP" is the extreme way to clarify sentences.)
Finally I point out that when handwriting with a pen or pencil - of course, extremely few people now handwrite multi-sentence messages for any reason - I observe that it is common (english speakers, writing) to leave a fairly substantial gap between sentences, much bigger than the word gap. So, there's kind of an analogy.
(On that note the only human being I have heard of who handwrites today, is the popular author Neal Stephenson who apparently writes by hand when writing a ms. one could sift through his paper ms and see if he leaves a bigger gap!) Thanks for the great question!