Definitely a passing phase for me.
I did a bit of gymnastics at school and then switched to Rugby about 15/16. I played that for 15 years up to semi-pro level and then gave it up as I kept displacing vertabrae in my back (T1-T3).
Then I got lazy and did hill walking for a few years then started running. Then a triathlon - think I am in 4th or 5th season now. Will I stop? Probably at some point - so definitely a passing phase.
I like road biking, running, mountain biking, have grown to like swimming. I love the adventure race stuff, although best done only now and again, not sure they would be the same if done every month.
I suspect many will come into triathlon and compete and then move into other areas as well. They will use the skills, I fancy the Dark 10k swim but it is enabled by the triathlete in me.
I can do an easy paced run, can I do an easy paced triathlon? Not really. I have similar scenario in swimming. I can do an easy breast stroke or crawl - I can't do an easy Individual Medley. So yes I think a Triathlete, in general, is either competing, wanting to compete (ijured, skint), or a member of triathlon club. Other than that they are a just a person who trains at running, swimming and biking. Simialr discussions to be had with runner vs. jogger (former strives to improve, races or acheives a certain pace depednign on who you listen to).
I hope to continue to train at all three for a long time it provides variety, enables training when you have sport specifc injuries. You can mix with more people. And for me one of the main benefits is when you look at any aerobic based event you think - I could do that (with the exception of skiing maybe).
Dart 10k River Swim, Scotland Coast-2-Coast, Mountain Bike the South Downs Way, and many many more events I look at. All great life experiences in my view.