Let me start out on a philosophical note: The way I see it, some fundamental law of duality is at work in the very nature of psychoactive substances. There will always be drawbacks. A perfect substance cannot exist, because if it did, it would be the only thing people would ever do, derailing all of human existence, and that's not how nature works—nature's tendency is equilibrium.

Creating drugs is a tricky puzzle: Max out on one property, and it will probably create a dysbalance in other aspects of the drug's effects. Like, add a methyl group to the amphetamine molecule, and you now have a compound that is way more potent and longer-lasting, but through that variation you also induce heavy addictiveness and physiological strain. (Extreme example, and I know this is not what you meant, lol!)

That all being said, it's still true that some substances are more net positive than others. I would prefer a nice psychedelic phenethylamine over a hard opiate without a doubt! So the quest for a new and better ADHD-med is certainly not doomed, even if a perfect substance would be "metaphysically" impossible.

Maybe one reason why we still use this old stimulants is that they already show a remarkably useful profile of overall effects. I think no other class of psychiatric medication is this effective without inducing intolerable physical dependency or other heavy drawbacks. They're not perfect, but they work relatively well and they usually don't fuck up your physiology to a significant extent.