Have they discovered ANY mutation that is NOT destructive? One, just one.
There are many genetic mutations that are beneficial (they have recently found ones that appear to make us smarter); and it’s always not so easy to tell, since single doses of certain recessive genes appear to be beneficial although double-doses are fatal (cystic fibrosis and possibly Tay Sachs). Most mutations are actually neutral, some are bad and rarely they are good.
The one (or several, or many?) that makes humans less lactose intolerant over time?
The “sickle cell” mutation is one.
Individuals with one allel are more resistant to malaria than those without the mutation.
There are many known instances of genetic duplications that are beneficial.
How about the mutation that allows primates to distinguish between red and green?