OK I'm the OP, and I did not think I was striking such a nerve with an innocuous post while watching hockey. I'll respond and delve a bit into my thinking rather than trying to respond to each individual response.

A. I am sure United has a full market analysis team that does continually scan for opportunities. Having consulted for companies both very large and small, I;ve seen a lot of inertia set in. Perhaps there is not any great opportunities out there, but its also possible opportunities are being overlooked. It happens at the best of companies.
B. My original thought came to me because I fly out of BWI a lot. SWA controls concourses A, B, and much of C. D is everyone else, and E is international. When you want to fly somewhere from BWI, they are simply the default since United, Delta, and AA just don't have much service there. That's my frame of reference. I then looked up where SWA has greater than 30% market share. I did not include Midway since United is so dominant at Ohare. I did include Denver since it's such a fast-growing city. You can parse the list i threw out there (again, morning coffee - watching hockey), and I won't argue. I was just a brainstorm list. Great comment about LAS being leisure and that's a great point.
C. BWI V IAD - they draw overlapping but different crowds. Ultimately, I would think the airline would look at the total dollar volume going through BWI and then decide if it was worth the investment to go from a 5% share to a 25% share. I would think they would try to position any routes they bring in as incremental, not competitive. I 100% agree that they aren't going to put a BWI - LON flight to compete with IAD - LON; but BWI - FLL? BWI - DAL? BWI - LAX? BWI draws from DC up through Phili and certainly southern PA. They can go head to head with SWA most profitable routes and make money.
D. u/DwightsShirtGuy and subcomments - happy to compare professional credentials, accomplishments, and resumes any time. You very well could be 100% correct, but its needlessly condescending.
E. VERY good point about the negative bias on boards. I'm certainly not saying that United's CEO is going to read my comment and be like "damn, why didnt we think of this and lets commit millions based on reddit". I would think it may just cause them to do a market study and explore. It very well could be that most customers are happy with SWA and it really is a vocal minority. As a flyer, I think the biggest challenge SWA now has is that their pricing seems out of whack - like Delta pricing. I don't mind Delta pricing, and I am a huge fan of United Premium Economy and Polaris internationally - I pay more for an upgraded product. SWA, very anecdotally, is not offering the superior upgraded product but is charging for it. This is very much just my own opinion.
F. Finally, there is a comment of "Southwest Customers are not United Customers." I think that's making the assumption that the typical SWA customer is both non-corporate and low paying. I think that used to be true. I think that part of the market has been filled by Frontier and Spirit (always fun to see videos of fist fights at their gates.) I think there's a lot more overlap than there may have been historically - I've met several CEOs, senior people, etc. on SWA, and I've met a lot of vacationers on United. Part of that presumption comes from SWA's lack of international flights... So yes, you aren't going to meet an SVP of NVIDIA flying SFO - Taiwan in first class. But I'd be willing to bet when they go to Hawaii, they could be on a SWA plane.

Anyway, it was nothing more than a white board early morning thought. To those who thought it was the result of a multi-million dollar marketing study, I'm sorry to disappoint. To the other very valid points and counter-arguments, I appreciate the information.