It's not impossible to have upgrades at 100 hours. It just won't happen if American thinks there's a chance that that seat will be sold.

I have an upcoming flight on Sunday for which I've been watching the seat map closely. It's continually had ~9-12 seats open in First. This morning, when I checked at T-96 hours, I saw that one seat in First was taken, and one seat in Main Cabin Extra was freed up. I assume that seat went to an ExP.

Complimentary upgrades have been getting harder and harder to get as the years have gone on and airlines have pivoted heavily toward monetizing upgrades; this isn't just an AA thing. Look at how Delta markets their MCE equivalent as upgrades for Medallions, for example.