I think you can’t look at the past 5 years and the features that are being floated for Java 21 and future releases and think fear of change is the issue—the language is now changing rapidly.
I like his use of the word should, because as we all know it really means won't. I should exercise more, I should stop trolling people on reddit. API designers should control their APIs.
As for the technical objections, I think the conflict-management rules hold the most weight, but this problem is no more difficult than any other in language design. "not overrideable" is a feature, not a bug (can we please give up on behavioural inheritance already?), and discoverability already sucks. I mean, I still use duckduckgo to find javadocs.