Why didn't Johnny mind dying?Besides the fact that he was content seeing the parents thank him and the children being alive, of course.
I am wondering if there is something that makes you think Johnny has other reasons than the ones you mention. He does not actually explicitly give any other reasons why he does not mind dying in that letter that he puts in the book that he gives Pony.
If I had to try to come up with another reason, I would say that, just in general, he feels like his death might have some value. In addition to the things you mention, perhaps he feels like he is helping to save Pony. Maybe he thinks that his note to Pony, and the experience they've had together, will make Pony realize that he can really "stay gold."
In the end, of course, it does help Pony. Johnny doesn't know that, but maybe that's what he is thinking as he writes that last letter.
Johnny tells Pony that he doesn't mind dying because those children he saved from the burning church were worth it. He feels that they have a brighter and more potential-filled future than he does, and therefore, his sacrifice, in the end, is justified.
Johnny's home life was also wrought with despair, pain, and hatred. The other greasers even go so far as to tell him that he "ain't wanted at home either." Hinton tells us directly and indirectly that Johnny's home life is abusive and violent. Therefore, his departure from life will be a bittersweet end to a lot of misery that he has personally experienced.