No, HDR brachy is not taking its place. I think the problem is economics. It is the lowest cost procedure, so doctors don't make that much money from each. It also has a steep learning curve and it's hard to do well until one has done a lot of them (like surgery). As with surgery, there is often a mentoring relationship with a famous, high volume practitioner until the neophyte comes up to speed. Unless he has a famous practice, a doctor may not do enough volume to justify the investment in training, and there are much more lucrative procedures he can specialize in (like IMRT). So the generation of UWSeattle brachytherapists like Grimm, Sylvester, Blasko, Wallner, Taira, and Merrick, is dispersed and may be among the last of them. There are still other good ones in the US (e.g., Moran,Zelefsky, Stone, Ciezki, Frank, etc.), but the practice is declining.
Other reasons for the decline include the fact that external beam radiation, especially SBRT, protons and IMRT, have improved and are now providing similar outcomes with fewer side effects than they used to. Robotic surgery has become the most popular treatment, and many patients never explore alternatives after their first urologist visit.
I don't want to overstate this. Brachytherapy (and it is predominantly LDR) accounted for 28% of all radiation procedures in 2012, second only to IMRT.
Trends in the use of radiation for primary (first-line) treatment of prostate cancer- Allen