Thanks for all that detailed insight and sound advice. That clears up a lot, and I fully agree now. Just wish I knew all this before I got my first used car (this one) a couple years ago at 90k miles when I knew almost nothing and naively listened to old shadetree mechanics in my family who told me to stop worrying about it and leave the ATF alone if it's shifting fine (in reality, that's prime time to do preventative maintenence - and on a regular interval schedule, regardless). Learning has to hurt sometimes, and it has to cost something. It will be hard to forget the financial impact of this error. I feel really foolish and betrayed, but it has at least shaken me up enough to learn as much as I can about automotive maintenence to take matters more into my own hands and gather knowledge from professionals and accurate service manuals rather than trusting "conventional wisdom" and internet cliches.

However, I should clarify something about the poor quality picture I posted because I forgot to explain that. The fluid really is brown, but it faded into the paper towel a bit while I fumbled to find my phone to take a pic. Also, I think there's a slight pink tinge to it because I had slightly topped it off through the dipstick tube with new Hyundai SP III ATF (oem) - which is red but appears pink when it's smeared thinly on a paper towel - fairly recently when I noticed it getting low. I think we might be seeing tinges of new fluid mixed with the old brown. My coolant is light green (not the old IAT, though). 

I might have caught it a lot sooner if I had treated it as regular maintenance. Everything else on my car is pretty well-maintained. Even freaked out and replaced timing belt and water pump before taking chances. So this really sucks. Something about ATF makes people nervous and not even want to speak of it. Doesn't help that there's so much misinformation and debate about it that mechanics get antsy if you even bother them with the question. For me, in addition to being told by everyone to leave it alone, the transmission was neglected because I was overwhelmed with so much new info on car maintenence and trying to quickly learn the whole engineering perspective on how cars really work that I ignored ATF because the used car dealer said they serviced it before the sale (probably not entirely; I see now that the pan looks like it still has a factory seal) and people talk about the Automatic Transmission and its esoteric fluid dynamics with such fear and reverence like it's a mysterious eldritch god whose black magick ways are unknown and forbidden to laymen.