But kids don't have to be in potty trained to get into daycare.

My kids learned the early academics at home, and certainly knew their letters, numbers, shapes, colors, etc. when they started preschool. They also had a least one mommy and me class per week, and at least one playgroup each week. Throw in a swim class, trips to the parks and library, plus shopping and errands, and they were quite busy.
However, I find preschool enriches them even more. It's the first place that they pick their own friends! Most of our playgroups consisted of my friends, and their children. It was interesting to see what children my kids were drawn to, what children they asked to play with outside of school.
I believe that daycare is one of the reasons why kindergarten is the new first grade. I do believe it is very easy to teach academics at home. However, teaching children how one behaves in a school setting is very hard to teach outside the classroom, and teachers do expect that children had been to preschool or daycare.