I've noticed the same thing with my 3d printing. I think that if somebody already has a way of doing it without the technology, then it's going to be a harder sell to use technology. There isn't much that it can do that one couldn't potentially do in other ways. I know there's stuff like vases created by rotating triangles, but the things that one might really want to do could be done in other ways. Which can make it a hard sell for some folks. especially those that aren't good with CAD software and unable/unwilling to learn.Perhaps but then again you don't need Arduino to use servo's for turnouts either.
For things such as you built, speedometers and weigh bridges and so forth, it seems great; however, for day to day things such as changing points, lighting - it does over complicate it.
As for the the other things you chose to bring up - your absolutely right, and I stand by what I said about farcebook and the hobby declining. Now I'll add this ... new technology rarely makes things better, it only creates more, over priced, things to fail and/or make people lazier than they have already become.
From what I can tell, most of the things that one might use arduino for on a layout are things that are optional or have an already existing option. I just picked up a kit, so I'm not quire sure what I'm going to do with it, but these seem particularly well suited to dealing with accessories.
My initial thought is to use it to run servos on my turnouts. But, I'm also likely to use it for lighting.
No arguments about FB, that's a terrible site and I avoid it wherever possible. The revelations of the last year or two just reinforce the sense that it's not a good idea to involve myself with it.