Not agree. it is not like to what you said. If you have common sense, and enough English knowledge, you can DIY absolutely, especially I-485. Most work of I-485 are filling the forms about yourself.
Now tell me, who know yourself better, you and your lawyer? Of course you. Some people delayed their cases just because they waited their lawyer to fill the forms, and corrected the mistakes. If you did yourself, you might avoid that.
You should learn DIY.