Overall, the 737 mid-cabin lav is a great idea. The issue of the blocked aisle is real. Too bad they aren't around as much as before. But I do recall reading somewhere that eliminating them reduces considerable under-floor plumbing and cost.
I've been begging, pleading, and arguing my case regarding mid-cabin lavs to the powers-that-be for years, particularly when a new aircraft order or conversion of existing options was announced. Each time, the explanation came down to one thing; resale value. AS buys aircraft on what they assume will be a 20 year life cycle with the airline, and with an expectation of a certain resale value to lessors and/or other companies when they renew the fleet. With the most likely purchaser of 738/739 equipment being tour operators flying in an all-coach configuration, the airplanes would become more difficult to sell with a fixed monument forward of the wing instead of all three main cabin lavs in the rear.
Oddly enough, they did concede that there'd have been little to no need for anti-tip tail stands if the mid-cabin lavs had been installed, since the CG moves further forward, but by then the fleet had been standardized with 3 in the back and there was no going back.
I don't think there's any argument, however, that a mid-cabin lav is an overall positive for the customer experience.
"In this present crisis, government is not the solution to our problem - government IS the problem." - Ronald Reagan
Comments made here are my own and are not intended to represent the official position of Alaska Air Group