Well it’s true that ADHD traits are on a continuum. That doesn’t make the diagnosis point “nonsense” - I would describe it as the ‘best guess we have so far’. Blood pressure is on a continuum too - anyone with blood pressure higher than a specific value is considered to have high blood pressure and anyone below it doesn’t. In practice anyone close but just below the threshold will probably still get advice from their doctor about how to lower their BP.
There is scientific evidence that ADHD brains are “wired differently”. Of course, you are right to say that evey person’s brain is unique, but this does not mean that there are not some typical differences in ADHD brains. Scientists have been researching this. For example: Doctors have known for several decades that dopamine signaling is different in ADHD brains One of the strongest pieces of evidence for this is that stimulant medications, that increase dopamine signaling, improve ADHD symptoms.
Other research on brain differences:
https://www.nih.gov/news-events/nih-research-matters/altered-brain-connections-youth-adhd.
or here https://pmc.ncbi.nlm.nih.gov/articles/PMC7880081/
there is is an overview which starts with “Regarding brain structure and function, findings from neuroimaging research….”
Your point about most people who are assesssed for ADHD are diagnosed is often used an argument that ADHD is being over diagnosed. You probably do not realise that due to the fact the the assessment takes several hours, there is a pre-assessment/triage phase which weeds out people. So, anyone who gets to a formal assessment has already had a fairly significant level of clinical assessment.
And yes the diagnostic criteria have broadened significantly and again this is also used as evidence that ADHD is being over diagnosed. This is a fundamental misunderstanding - the criteria have broadened due to the increased understanding of ADHD, which is the result of scientific research. For example, pre 1990, very few girls were diagnosed because most of the research was on boys so there was little understanding of how ADHD presented in girls. This does not make it incorrect to diagnose girls. It is an evolving story. I’m sure in a few years time when there is more understanding, the diagnostic criteria will be changed again - and who knows, they may even be narrowed, moved sideways or expanded.