And why should tinkering be easy ? The whole point of it is understanding things beyond the initial use and this is the mindset that, I believe, only handful of people have (like one in ten).
I distinctly remember that when I was tinkering in high school with things most of my peers were tinkering with people (a.k.a living life). So maybe, just maybe, it does not really matter if tinkering is easy or not - because its mostly character that decides if You will be into it or not (I subscribe more to nature then nurture side).
And I honestly question if my children should waste their life on things instead of people.
Let me know if you ever figure out - I feel I'll be facing the same problem in the next few years.
From my tech person perspective, I agree with your summary: advances in computer tech and consumer product design have done away with tinkering: where it isn't actively discouraged, it's still hard and doesn't seem that fun anymore.
Yes, people will bring up BBC Micro and those 8-bit handheld boards and Arduinos - but the truth is, even if they replicate the experience of 20-30 years ago somewhat, a big part of the motivation (to me) was knowing you're doing something unique, that you can't just trivially buy a ready-made product that does the same thing but much better.
I feel you can't get people to "tinker" in general - this happens when they have a goal and tinkering is the best way to achieve it. For me as a kid, this was building model rockets and writing video games - it's what got me to do electronics and programming. Without such goals, I wouldn't sustain my interest in learning and playing with these things.