As someone who has felt strongly about T being a mother figure and having to be kicked in the a s s proverbially regarding the matter, here's my opinion.

I feel in some ways that a T can take the role of a parent. However, where many people become stuck is this; when people want their T to keep the role as a parent with the client remaining in the role of an infant for an indefinite amount of time.

I'm going to talk about my definition of a "good" parents, but in the singular:

The parent starts out by meeting all the needs of the infant. As the infant becomes a toddler, the parent encourages him/her to grow, sometimes away from the parent. As the toddler becomes a small child, he/she will begin to demand independence. The good parent encourages this. He/she begins to develop relationships away from the parent. By the time the child is school-age, he/she develops more friendships and has to cope with not seeing the parent during most of the day. They interact with other people with nurturing roles, such as the teacher, or school nurse. The child will increase that independence and the parent remains in a supporting role. The parent is there when the child needs them (for support or discipline).

That is how I see my relationship with T now. A "good" parent only wants what is best for their child. T has described our relationship in that manner. She only wants what is best for me, whether it hurts me, her, or both of us. It took me a long time to get out of the infantile stage. Now that I am in the child stage and T wants me to handle certain things on my own, I am actually happier (despite going through the hard period of change) than I was before.

Just my two cents.