Data points:

$139 for BNA-DEN and $99 for DEN-SFO yesterday. At this pricing it is the equivalent of an E+/C+ seat fee on UA/DL though of course you are not guaranteed extra legroom nor do you get a reserved seat.

As I had a C boarding pass I decided to cancel WN rather than pay the upcharge (free points redeposit) and instead booked a UA nonstop for 15k that included an exit row seat. It ended up being the right decision as all the DEN flights were delayed.

It will be interesting to see if the seat rez fees make up for the lost revenue from EBCI and Upgraded boarding. I am sure the beancounters ran the numbers but WN has a lot of passengers that don't seem to care and may simply go with Basic fares or not at all.