You'll find no shortage of UA flyers who rue the day they introduced UDUs. While I'm not one of them, if you did any mix of dom/int'l, you likely had more than enough CR-1s and e500s to cover all your domestic UG needs (and had a better chance of clearing).
UDU also penalized those who "knew the system" in favor of a more generalized process - bad for FTers, good for joe schmo 1K who somehow didn't realize he/she had a stack of unused UG instruments.
You're a bit off on your perceptions of UA F pre-UDU by the way - majority was always filled by paid/UG pax, NRSA and empty seats were the minority.
As always, YMMV.