So you could argue that if you are on the boundary, whether you have ADHD traits to the extent of being a disorder depends on the environment you find yourself in, and so you could characterise that as some aspects of that environment 'giving you ADHD', but it's still not yourself changing.
Deficiency, since it gets in the way of successful, effortless living.
People say they "suffer" from ADHD and use it as an explanation why some things harder for them than for others, etc. Many complain. People classify it alongside other mental health issues.
It's a bug, not a feature. Would anyone without it wish for it? Would anyone with it and given the choice for it to be gone choose to keep it?
Balance. The inability to "focus" attention at the likely relevant thing as a matter of course is suboptimal. You don't want to get distracted by the wrong thing when it is a matter of life and death.
I don't know that your theory is any more likely than a theory of ADHD being less likely in those before-times because it would get you killed because you're not paying attention to the right stuff and nature then stomps on you. Not that I have any reason to believe this idea.