What makes you believe that dextroamphetamine is not neurotoxic at therapeutic dosages?
Dosages of dextroamphetamine that produce neurotoxicity are much higher than those used in human therapeutics. In animal experiments dosages are between 20-60 mg/kg injected. That would be 1.2-3.6 gr of injected d-amphetamine for humans. In animals, these dosages produce neurotoxicity but also death caused by hyperthermia of most specimens. These effects are not seen in humans.
In science, proofs must be shown in positive. There are no data to suppose that d-amphetamine is neurotoxic at therapeutic dosages. Human dosages are 1000 times lower than neurotoxic dosages. There are no data also long time d-amphetamine exposure causes neurotoxic damage, and amphetamines have been widely used and studied for over a century.