They used to never charge extra fees, but then added the "board first for an extra $10" fee (that comes with a free drink). I bet they will figure out ways to find some more.
I think optional fees are a lot different than a surcharge. If traveler wants to pay $200 to sit in United Plus, or $600 to upgrade to business, or buy a day lounge pass, etc. those are all options which bring some value to the customer (in the case of the upgrades sometimes a good savings as well) and are entirely optional.
Charging $200 online for a ticket and then hitting the unsuspecting consumer for another $50-100 at the airport in various fees isn't the way to build brand loyalty.
Carriers need to be looking how to upsell and differentiate which WN (which I rarely fly but like when I do) is doing.