Cobol had 10x fewer developers at its peakc and probably 100-1000x fewer lines out of code out there, in the wild. Plus it didn't have a huge Open Source community.
The cool kids of the future ain't going to learn Java once companies move to a better technology. It may take decades, but eventually people will stop building new code in the language.
a) Java is a bad technology and it's not moving forward ("once companies move to a better technology")
b) new Java developers aren't being trained, including very young and bright ones, straight out of universities, and they don't enjoy Java
Both claims are false.