The phrase "became threadbare" means that something has become worn out or frayed. Its antonyms would be words that describe things that are still in good condition or appear new. Words like pristine, immaculate, flawless, perfect, and untarnished could be antonyms. Other possible antonyms include durable, sturdy, robust, resilient, and strong. These words describe things that are capable of withstanding wear and tear, and are not easily worn out or damaged. Using these words can help paint a picture of something that is the opposite of threadbare and convey a sense of newness, strength, and durability.