Getting the Ice Beam would've harmed or maybe even killed Samus at that point. When she absorbed the SA-X at the end of the game, it restored her DNA to its original state and allowed her to use the Ice Beam, or at least reconciled the Ice Beam with the Metroid part.
Q: In Metroid Fusion, it seems to have established that "Samus can't use the Ice Beam because she has inherited the nature of the Metroids", but why at the very end is she unconcerned when she regains the ice beam?
A: When she absorbed the core of the SAX (the perfect condition of Samus), she basically reconstituted her genetic condition. She didn't recover the physical damage of her amputated suit, but she did recover her genetic condition.
This is Sakamoto answering the question, mind you, but at least it's something resembling an official answer.
Metroid Dread will just be short slip of Sakamoto peeing directly into a camera and therefore on you the player on loop for two and a half hours. Upon completion he turns around and squats down and there's a freeze frame with the text "See you next mission!"