I didn't feel this game undermined the end of FFX at all. No matter what ending you get for this game (except the bad one), there is one thing that remains the same, Yuna changed. Yuna was able to become the person the she wanted to be, not the person she had to be, but wanted to be. And that is all because of Tidus. Tidus had an impact on her life that extended past his passing, past his presence. The mere hope that she could see him again, the mere hope that she could have the life she wanted, changed her perception on the world, and changed her. She didn't change for Tidus, she changed because of Tidus, and no matter if she finds him or not, she ends up with a life she wouldn't trade anything for. I think FFX-2 personifies the importance that Tidus had on everyone, especially Yuna, and greatly hightens the impact that the ending of FFX had.