"There’s a reason adults don’t pick up Japanese or learn how to kite surf. It’s ridiculously hard. In stark contrast, young people can learn the most difficult things relatively easily. Polynomials, Chinese, skateboarding — no problem!"
Um, ask some teachers about how easy that is sometime. I'm sure you'd get an earful.
The biggest difference between adults and children is not neuroplasticity--the biggest difference is TIME.
Children can spend 4+ hours a day learning a language, playing guitar, drawing, etc. In one year, that's roughly 1,000 hours.
I would challenge you to find an adult who can commit 1,000 hours in a year into learning something completely new.
In addition, children have a much better reinforcement loop. A child who can say even a couple of words in a foreign language will get wonderful praise; an adult won't receive praise until he can practically get a PhD in the language. Musical instruments are similar--a child can bang out stuff and everybody thinks it's wonderful--one wrong note as an adult and everybody is a critic.
I suspect there is some neuroplasticity involved in language--certain sounds seem to be indistinguishable to non-native speakers after a certain age. However, I suspect that the neuroplasticity is dwarfed by simple time.
I see little value in futzing with drugging the brain until we normalize things like time.