I won't say I'd complain if they did, but for many of the reasons already said by everyone here, they wouldn't fly very well. It's true, they don't have NEAR the name recognition that Anastasia does, and I do like the distinction that they were very much marketed towards young children. Anastasia works for many markets, same with Prince of Egypt, my other favorite animated non Disney musical.
I love both of them regardless of the target age however. If they were to be made into stage musicals, I think Fan123 is right, SP is the more salvageable of the two. Odette may have the usual princess qualities of soft voiced, sweet natured, etc. but I always appreciated that along with that, she had a mind of her own and wasn't afraid to use it when necessary. She attempts to take charge of herself, but was kinda hindered by that usual fairy tale spell. Thumbelina would be harder in terms of stage design (obviously) and I think the story itself flows better, but it SP has the more salvageable story to make work on stage possibly. And believe it or not, I don't think Odette/Odile would be a dual role unless they were to cut Odette trying to get Derek's attention at the ball that's it's not her down there (Yes, I''m a nerd who knows this movie well ;P). We see both of them, and we should if we're to keep the gender politics of 2019 in it because that is one of those moments where she's trying to take charge of her own fate and fight against what's happened to her. Without that, it would the ballet, where she's absent from that, and seemingly just lets it all happen.