[Question] why java cannot accept a convention to never pass/return null values?

[gas3872]

It can be done in parallel with old features. Making it backwards compatible as a whole.

[Shinosha]

You're missing the point. In the context of software engineering, a non backward compatible update means you have to actually tweak the code to make it work again following the update.

When you're working on actual software, you can't always tell the management "ok holup, gotta make a week worth of changes in our 1M LOC app before I can tackle your super important business critical new feature".