There may be another reason that lugs have persisted so long on "very lovely and high end frames" as you point out: aside from them being very attractive, people with enough money to afford "very lovely and high end frames" are usually older and came of age when lugged construction was considered the best way to build a frame. Welded construction was used only on the cheapest department store bikes that weighed 40+ lbs. Fillet brazing was not as widely known and while used on many fine Schwinn frames, it was often confused with the electro-forging process used on the cheaper Varsity/Continental models. So if you are a well-heel gentleman (or lady) of a certain age and you want to buy a high-end frame, there is a very good chance that you will look for a steel frame of lugged construction because that's what you grew up with and are familiar with. And frame builders know their customers and will build accordingly.

Time will tell if in 20 years the cyclists of the day will still be interested in lugged steel or if cyclists who grew up with TIG welding and carbon fiber will prefer that methhod of frame construction.