The catch is that unmedicated ADHD outcomes aren't good.
I worked with the same group of students for a 2.5 year period of pre-K and kindergarten. Low self esteem, anxiety and depression and to a less extent conduct disorder were plain to see in the majority of the low executive function students by the end of Kindergarten. I saw these negative outcomes develop as a direct consequence of difficulty managing behavior in class and keeping up with peers.
Particularly for kids with combined inattentive hyperactive ADHD symptoms it's nearly impossible for them to get through the day while keeping up academically AND keeping their behavior inside the realm of "acceptable classroom behavior". Lacking a robust support system for students with extra needs (think an additional teacher or teaching assistant in classroom at all times) there is a very finite limit to how much you can assist without creating issues for the progress of the class as a whole.
The reason that I am careful to use low executive function (EF) as the label at this age is that even for experts in this area it's incredibly difficult to predict who will "grow out" of their lower than average EF issues and who plateaus with maturity.
I think the current consensus that diagnosis and especially medication for ADHD is too difficult prior to 6 or 7 is correct. I've seen too many students have seeming miraculous gains in EF and catch up with peers in a matter of weeks to think that preschool is the appropriate time to diagnose and treat ADHD.
The students who continue to lag behind in EF are substantially behind in basic grade level knowledge when they set foot in primary school. The amount of catch up they have to do is substantial even for neurotypical students by time that a formal diagnosis and medication is an option.
Now add in the fact that a large portion of the parents of ADHD kids have a parent with ADHD or less than average EF skills. They are less able than most to give their children the extra out of classroom help they need.
Getting kids on medication ASAP once a diagnosis is confirmed and a well tolerated treatment is found is a no brainer. By the time that this comes into play you are already in educational triage. We're talking about 1st graders that can't read simple consonant vowel consonant words in some cases.
The obvious solution is putting in a low EF safety net in pre-k and kindergarten. Extra teachers in classrooms, extra help with literacy. Making sure all parents are aware that their child is has an elevated risk for ADHD diagnosis later down the road, so they can familiarize themself with the diagnosis and treatment options-- and more importantly so that people are assessing and testing to see if they catch up in EF function.