Is the weaning working too well...

My twins are eating 3 meals a day- no snacks yet- and it's going really well. But now they are only drinking like 16ish ounces a day. I know formula is still so nutritionally important before 12 months. I also know their birthday is not a magic day. How do you balance this? I feel like they would drop formula completely if I let them (I won't). The pediatrician said it was fine when they went from 30oz a day to 20 at like 8 months old. Is it okay that it keeps dropping?? FTM.