I still put two spaces after a period and am vastly annoyed when some whippersnapper editor wants me to only use one. For some of us, that's an automatic thing.
I still use the serial comma, too.
Ditto. The point of writing is communication, and whatever makes comprehension easier to the reader has value. Spacing is a visual cue, as is punctuation. There's nothing "19th century" about two spaces after a period (in fact, it was taught up through most of the 20th century and the rebellion by lazy students (similar to the refusal to use upper-case letters) only caught on in the internet age with concern about the number of characters.)
I never could see any reason to put extra spaces after a period, so I never have. Fortunately, I don't have hidebound editors who are still living in the 19th century to deal with.
For many, it is easier to read sentences with a little more space between them. (You could always learn to read ancient Greek, which was originally written without punctuation or spaces, and see how that feels.)
Or, you can go on being smug about making it harder for many people to tell a period from a random dot on the page.
It was an artifact of typewriting that's obsolete (or at least obsolescent) in an age where we're no longer tied to the physical limits of the way a piece of hardware is built. But generations of people were trained to do it, and if it makes them more comfortable, so be it. It's an utterly unwinnable fight, IMO, and I have equally unwinnable fights that I'd rather devote my energy to (like trying to persuade people that it is almost never correct to form a plural by adding apostrophe+s).
For the record, I'm a big fan of the serial comma.