This feels like arguing against a straw man. I don't think anyone generally says that a brain can't change once you're an adult. If that were the case you wouldn't be able to learn anything at all. Clearly we all can learn as adults so there is neuroplasticity as adults.

The general argument that I hear which is true (from all I've seen and read) is that neuroplasticity is greatly reduced in adults as compared to children. There is a period in early childhood where brains are incredibly flexible and adaptable, and as they age they continuously lose that ability until the majority of it has been lost by adulthood.

It doesn't mean there is zero in adulthood, just that it is very small compared to childhood.

I think you're arguing against something nobody really says.