The phrase "goes soft" typically refers to something that loses its strength or becomes weak. Its antonyms, on the other hand, would be the opposite of this - words that describe strength, durability, and resilience. These include terms such as "sturdy," "resilient," "robust," "powerful," "strong," and "stable." Such words speak to the qualities of an object, a structure, or a person that can withstand adverse conditions, pressure, or stress. Using these antonyms instead of "goes soft" can add a positive connotation to your description and reinforce the idea of strength and resilience.