I thought United Airlines would ignore my letter, but they did not. Of course, I sent four copies to four different departments, so that may have had something to do with it. Irrespective of the why, last week, I received several letters of apology, both of which explained that it was not dictated by policy, but rather by pilot prerogative whether or not to wait for passengers. One of the letters said (and I'm paraphrasing): We hope you allow us the opportunity to earn back your trust.
I hate to be swayed by mere words, but must admit that those words did sway me. Well met, United Airlines. Well met, indeed! It didn't hurt that they gave me some bonus mileage and a few vouchers :).
Needless to say, I am pleased. And I shall happily fly United again!