I don't necessarily think that the lower interest in DIY is Head-Fi's fault as much as it is the market. Yes, Head-Fi has forbidden Group Buys. That said, I understand their position and they probably didn't have a choice. Group Buys evolved into making large buys of headphones, cables, and other things besides just DIY. A few of those resulted in the organizers simply disappearing with all the money.
I remember having a conversation with Justin of Headamp at a meet a few years ago. He said he never had an issue with theft or dishonest transactions until he started selling headphones themselves. So maybe there's something about a huge market and items of relatively small physical size that attracts people with less than honest intent. In the end, it was probably way too much to expect Head-Fi to allow Group Buys for DIY but forbid it elsewhere. Heck - no offense to others reading this thread - but some of the mods around here probably don't know the difference between a real DIY product and one that's not.
As for the overall market and DIY - I've noted and stated this before. I've been heavily involved, on and off, with the model airplane industry since the 60's. (To be clear - I was just a kid back then!) For several decades, the only way to get a really high-performing RC or control line airplane was to build it yourself (kit or from scratch). Readily built planes simply did not exist. There were some plastic offerings (Cox, Wen-Mac), but they were really just toys and mostly pure junk. Then something happened beginning at the turn of the millennium - labor from China/Asia was incredibly cheap and suddenly became available world-wide. Within a few years, kits and parts for scratch-built designs became scarce. "Real" kits have almost disappeared from the market. Instead, you have ARF's (Almost Ready to Fly) and RTF's (Ready To Fly) available everywhere for cheaper than you could buy a kit and put one together. That pretty much left DIY to the highly-experienced, design-and-scratch-build folks. The masses left the DIY culture.
I think the comparison is completely analogous with headphone electronics. Schiit and equipment from China/Asia is so much cheaper than anyone can build, it's removed all but the most experienced DIY-er's. They in turn, have little interest in putting in effort unless it's for top-of-the-line performance. Or, some have left completely and are custom-building on their own or manufacturing premium designs. This has the effect of shrinking the entire community, as no one is really reaching out to beginners anymore - unless it's for something outrageously simple, like cables and blu-tack. Sad, but I guess it's a natural evolution of the market - as with others before.