Just a data point:

I flew EWR-MCI on Sunday night and though Y had a handful of empty seats, the app claimed it was oversold when I went to generate my BP. I had the following offers:

Miles: 12K/14K/16K
​​​​​​​Money: $600, $700, $800

I ended up having the entire exit row (A319) to myself, so not sure why it thought we were oversold. It had not been booked full the couple of times I checked prior to departure. Obviously in this case, had it been over, I would’ve taken the money over the miles..