The term "more well-developed" is subjective and can vary depending on the context in which it is used. However, some antonyms for this term could be underdeveloped, primitive, rudimentary, or basic. When referring to physical or societal structures, antonyms like dilapidated, decayed or disreputable would be appropriate. In the context of human qualities, antonyms such as unrefined, unsophisticated, inexperienced or ignorant would be more applicable. Ultimately, the opposite of "more well-developed" will depend on the specific scenario in which it is being used and should be chosen accordingly.