Even if the benefits you imagine are realized, that does not necessarily result in business value.
“More maintainable” is not a value proposition unless it means “we have important bugs we can’t fix now” or “we have important features we can’t implement”. Usually “porting” or “rewriting” means feature Y will be broken while you have feature X disassembled on the garage floor.
Note that “avoid adding new features” can itself be a feature.
I agree that starting small is useful in mitigating risk.
Migrating to modern Java and JDK might be more effective.
The fundamental conditions that led to the current situation are unlikely to be solved by choice of language.
If a decision maker wants to rewrite a code base, then it gets rewritten anyway.