Well, I built the BYOAC of my dreams in 2012. It came out spectacular and I thoroughly enjoy it. I also have a room scale VR system and do console and high-end PC gaming too, but the arcade machine fits into the mix seamlessly, gets played regularly, and I don't suspect that will change with any new gaming tech that comes along. There's no question that nostalgia played an important role in my building the machine. I was in my wonder years during the late '70s and early '80s, so that happened to be a big part of MY motivation to build. But I do have to say though, that while I once seriously considered building a second machine, the first one has been so versatile that I doubt I'll ever build another--not because I've lost any interest, but simply because I don't need another and don't want to have to maintain two. Yeah, the guts continue to evolve, and I still need to replace parts from time-to-time, but the cab itself is so solid that it should last the rest of my life, literally.
I too have noticed how the traffic here on the forum has slowed in the past few years; but honestly, this is one hobby where archival posts will tell you almost everything you need to know. I don't think this hobby is dying, but rather that it has just matured to a point where if you were "of the age" where nostalgia was the chief driver, you probably already built your cabinet a few years ago during the "surge" when all we late Boomers and early X'ers reached middle age and found a little extra money and time to spend on our mid-life crisis projects. If you are younger, however, you are almost certainly part of a much smaller group of arcade enthusiasts who are not driven by nostalgia as much, but more by a true love of classic gaming. The cool thing about this is that if you are in this category, you might actually be more of a "pure" arcade game fan than some of us older folks who can't quite decide whether we like doing this because the games of that age really were great and timeless or merely because they "take us back". This is why I think that if arcade cabs are going to continue being built by future generations, it is going to have to be due to a continual re-discovery of what makes arcade gaming on dedicated cabinets so unique and not really possible to fully virtualize.
One thing we can be sure of is that you can't pass nostalgia on to the next generation--they're busy making plenty of their own. But if classic gaming played on dedicated arcade equipment in the home is able to be recognized as a worthy form of entertainment in and of itself, independent of the nostalgia--like say, table tennis or pool or foosball, which I enjoy but have no nostalgia for, then the future should continue to see classic arcade cabs as desirable gameroom fixtures for the foreseeable future along with people building and buying them. Sure the traffic here has slowed, but I don't think it's death; I think its just what the new normal in this hobby probably looks like.