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.

There were 10x fewer developers in general back then. And who's going to take over that community once all its current members retire?

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.

You're directly or implicitly assuming that:

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.

Exactly. Even-though my livelihood right now comes from Python, I was trained in Java in my university(along with C). I did some coding(web scrapping) in Python but that was it. I still can and willing to write in Java.

Banking and Government are still making services in COBOL . It’ll never disappear.

There are a lot of Cobol replacement projects underway. Yes, it'll still be around for awhile, but there's less of it in the wild every year. The sad thing is that many companies prefer the mainframe Cobol service as it has run reliably for decades and the Java replacement does not have that trait.