I agree that the sea change in global culture related to technology has dramatically changed those who have been immersed in it from childhood into passive observers rather than active participants. Playing video games or Wii is not really actual participation. I think you are right when you point out that sports like dirt biking, skiing, fishing, hiking, etc. are dealing with the same decline in actual participants.
Much of this is, as you point out, due to the revolution in on-line computerized activity and the behavioral changes this has created in younger people, especially millennials. It's an extension to a frightening degree of the passive group tendencies I alluded to in my earlier post.
I use computers extensively and have for decades, but I do not play any of the games so popular these days. I'd rather be out doing things. Real things. My wife, an IT engineer, has a smart phone, but I detest the thing, and am quite satisfied with my $12 (on sale) Samsung that does nothing more than function as a phone. I use it primarily as a safety fall back, so it's usually turned off. I use a $15 per month pre-pay plan (Verizon no longer offers it) and have accumulated 1000 unused minutes, the maximum the plan allows.
I think excessive texting has had a profound and destructive influence on society, and is creating something akin to a human hive, insects quietly buzzing to one another, increasingly oblivious of the outside world, the real world, and especially the non-human natural world. Perhaps that's inevitable as the natural world vanishes and electronic simulacra provide effortlessly accessed substitutes. Effortless is a key element in all of this.
One element that has not been mentioned in connection with the smaller number of young people involved in sports like scuba is the effect of the baby boom and the proportionally much smaller numbers of young people in Western industrialized nations. The growth in sports like scuba reflected the youth explosion of the 60s and 70s, as did college enrollment and other areas strongly related to that youth explosion. As the population in these nations ages, as the youngest baby boomers move into their 50s, the activities that expanded to meet their needs have changed from scuba and skiing to medical care, plastic surgery, and retirement investments. The kids play on their computers and their smart phones and the economy rolls merrily along -- or does not, as the specific case may be.