"I'm worried they're going to sell the last upgrade!"

Then buy it. I watched the #1 get bumped to #2 SFO-OGG yesterday morning with 23/24 booked. After a while, #2 's name disappeared from waitlist before flight (and didn't appear on "upgraded" list) and ended up sitting next to me. He bought up. Your chances of upgrade depend on how long you last at #1 and the strength of the desire of those below you (or not even on the list) to move into that last seat.

Again, the waitlist shows only those who check in. There still may be more who might trump you - #2 yesterday would have ended up in the #4 position at door close if he hadn't bought up.

I personally view being #1 on the visible waitlist with one seat left as less than a 50/50 chance nowadays.