Sheldon had to give up on string theory and share credit to finally realize his dream. That’s two things he really didn’t want to do. Penny was not a hard working waitress, and while she did put some effort into acting, she wasn’t that good at it. She seemed to be wanting to be famous more than just be an actress, and when she got a form of fame for a part she did mostly because she needed the money she decided there were other jobs to get money. She also learned that she was smarter and more capable as the series progressed. If as Young Sheldon’s final episode implies Penny babysat Sheldon and Amy’s daughter, and somehow encouraged the daughter to pursue acting, it seems likely that Penny has achieved some success in that field after the end of Big Bang Theory, or her stories of acting would have discouraged the idea of becoming an actress.