The first אוקימתא of the gemara explains that above 10 we would be allowed to extend a hand to reshus harabim. Rav Meir explained that this is because above 10 tefachim the hand is a makom petur and not a karmelis. This assumption is not rejected as the gemara continues in subsequent אוקימתות. Thus Tosfos indeed says that above 10 is מותר לכתחילה and thus if we have a problem where we actually stuck our hand out to רשות הרבים, we can pull it back in by doing it above 10 tefachim. The Rashba does not accept this since as the gemara progresses we move to a simple קנס not based on the hand being a כרמלית. Therefore, it would be assur to extend our hand above 10 tefachim to prevent us from possibly doing it below 10 or because we may drop the object. (Rashba there addresses why it is not gezeira legezeira).
The Rambam does not mention the difference between above and below 10 tefachim explicity so we would think that he agrees with the Rashba. However, the מגיד משנה points out that the Rambam uses the language אויר רשות הרבים which by definition is less than 10 tefachim, and then he would agree with Tosfos.
The Mishna Berura also says like Tosfos and allows you lechatchila to return your hand above 10