Apparently, your friend got the wrong information. Rainbow fish are not extinct and are not difficult to find. However, the existence of urban development has a negative impact on the fish population where urban development only provides a small limit to the fish environment.

Well, if the maintenance is at the medium to difficult level. This is because rainbow fish come from the wild. So, even if kept in a large aquarium, these fish are not comfortable and easily die.

Hopefully my opinion can add to your insight!