I'd be careful about this. I've never seen anything to suggest that we could screen every child with FMRI scans to catch blatantly obvious indicators of ADHD or use scans as a definitive test for who should get medications. I honestly wish it really worked that way because it'd end all the hand-wringing about overprescribed medications and prevent a lot of kids from struggling unnecessarily, but unless something's changed pretty recently there's no one test that identify someone with ADHD.
Do I believe I still have ADHD now? Sure I "have it" based on how the word is commonly understood. But the thing is, in practice, almost everybody I talk to from friends to doctors to family has a different set of labels they think of me as having, and I change which labels I use to describe myself to accomplish different goals, and deep down I don't think anybody really has a clue what they're talking about.
Do you believe you have ADHD now?
I was fortunate to have some FMRI (I believe that's the tech) work done years ago, shortly after I was diagnosed with ADHD. The imagining made it so blatantly obvious that my brain was different than normal brains. Where a normal brain would light up like crazy for a given stimulus, mine looked like a black hole.
There are certainly bio/physical differences.
That said, I came into this comment thinking I'd take the stance of "of course it's a disease," but I think I've warmed up somewhat to what I think is your argument: lets stop with the labels.
What holds me back from getting on board is that it's not like a headache. I can't just be like "oh, I'm feeling scattered" and take a pill when I need to. That "I'm feeling scattered" is 100% of the time.
So I'm still in the "it's for sure a disease" camp. But I really don't care all that much in the end - I just want to make sure people stop hogging what allows me to even have a job and family.