The higher or lower learning curve for a language has no impact on when you stop learning, only on when you stop learning the language (or making significant gains in learning the language, since there's likely some corner you still don't know or know well).

If you stop learning just because you've mastered a language and learning the language was not your actual goal, you've failed. You should continue learning by applying the language to solve problems, which, unless you solve the same things over and over again, offers plenty of opportunities for continued learning.