I think the reduction of attention span in children is a combination of what you mention here but also a reflect on how education has changed over the years, when you where a kid, did you pay attention because you wanted too or because you knew there would be consequences for breaking the rules?
Positive reinforcement works wonders to help on this but put too much and you get a spoiled kid, some sort of punishment is required when breaking rules but parents don't do it, I'm not telling you to hit your kid, this ain't the 60s but parents these days ground their kids and due to kids begging and parents not having will power, they concede! I wish my mom was like that when I was young, but she once grounded me for 3 months for raising my voice against her... these days I've seen kids shouting at and hitting their parents and they do nothing... which then makes the kid push for worse behaviors.
Absolutely. I think this post's advice applies more to project-oriented tasks, getting things done. Positive is best when it works. But in general for behavior, some lines must be drawn. And those consequences need to be communicated clearly.