One thing that doesn't get brought up is that there is a difference between typewriter and word processor. One space works best in proportional fonts with typeset work. Two spaces work best in monospace fonts like text editor. The reason is that in typeset, the space after periods is wider than space between words, while in monospace need to put extra space to distinguish sentences. I think that some of the conflict comes from when and where people learned the rule.

Before computers, people used typewriters and were taught to use two spaces after period. Typesetters knew to use the right space. Early computers were mostly monospace. Word processors introduced variable fonts and were smart enough to collapse two spaces. Email and text editors were monospace. The web made everything proportional and collapsed two spaces to one hiding any difference in source. One space has won enough that people use one space in monospace text.