Yikes, "reduction in service" sounds right. (Though I'm also hoping that OAK-LAX is classified in the 350+ length...)
As others have said, WN seems to be able to get a beverage service to everyone in coach, even on short hops (like the LA area to LAS), so it can be done. I've heard FAs make the announcement early for anyone wanting a drink to be ready or to look at the menu - no asking "what do you have?" or you'll be skipped! I'm guessing that taking orders (instead of using the carts) lets them go faster (plus on shorter flights they'll usually only give out peanuts not really give a choice, saving a couple seconds per passenger).
I forgot that WN also passes out peanuts (and sometimes has pretzels) on short hauls in addition to the beverage service. If WN can do both a peanut and beverage service then DL should be able to do a full beverage service in C+ and at least a limited beverage service in C-.
On WN having the drink orders ahead of time and no carts has to make the service easier for them. I've been on WN flights before where NRSA's would be on board and help pass out peanuts as well which I think is kind of cool when you see a off duty pilot or FA flying home and helping the FA's pass out peanuts and talking to Passengers.