I agree with Kavan, you're fine.
Each extraction represents a risk of infection, nerve damages...
I don't understand your dentist idea : if he removes the lower WT, he has to remove the upper WT too. Without a tooth below, they will go down.
I have my both upper WT removed for a small cavity in one of them and it was a huge mistake.
Cavities can be fill, whatever the tooth. You can even have a root canal on a WT.
So keep your WT, your jaw line and your adult's mouth (having a mouth of a 12 years old is not fun), and find someone to fill your cavity.
If your chin bothers you, you can have a genio without removing any teeth.