She got the belt to get noticed, but a feud with Lita or Trish doesn't need the belt to be over. The belt made her credible, and feuds made her an asset.
But the WWE hype helped her in TNA more than the TNA hype helped her in WWE.
But, if you main event a TNA show, and no one is there to see it, did you really main event?
There is my tree falling in the woods argument. There is no denying that she did great things in both companies. But, Gail Kim wants to be known as a wrestler, but to TNA fans, she is best known as a valet. That's her problem. I bet if you asked her which was better for her, she would say that WWE best showcased her wrestling skills, and that was better for her career.
But this argument works for me as well. Vince doesn't need to recognize TNA. Anyone's career would better thrive in the WWE. AJ Styles could put on matches half as exciting in the WWE, but would be twice as famous. The only person who has been in both companies that has had a better run in TNA is Matt Morgan.
Let's look at TNA as a whole. I am a fan of the company. I love the in ring action, I like the stories, and I love that they don't hotshot every feud in your face. That being said, look at their biggest stars. They may get more TV time than they did in WWE, but it is the WWE that has allowed them to thrive. They trade off the names that they made in the WWE. They may be featured on TNA prominently, but they are thriving based on their time in the WWE, or WCW. The big company is star maker, and Gail Kim is no different. Would TNA have pulled the trigger on her run had she not had success in WWE? We may never know for sure, but contextually, success in WWE is what makes you a star in TNA, showing that people thrive based on the name they make at the big boy.