Learning how to read music and play the piano is a very detailed process and combines a lot of different skills and new information together. When you are learning how to play the piano, you will need not only email support, but you will need a variety of other ways to get your questions answered. It is true, that a well written course covers many questions, but having real live support can help clarify specific things where you are not certain.
Why is it important to have a variety of ways? Why isn’t email enough? It’s a natural assumption that when you are online, email support is the only support available. That is completely not true. You need a variety of ways, beyond just email, to get your question answered because everyone learns differently. Some people are perfectly fine with email, others need additional help such as video demonstrations or audio support. I have found that many students like to get answers from other students who have just recently experienced learning the same thing. Still others, like to be supported by taking little assessment tests. So, as you can see, getting good support in a variety of ways should be a strong item on your list of things you want in an online piano program.
IN addition to that, you should ask How long should you expect support to last? I believe that at the very least, unlimited Email Support should be a lifetime support as long as you are are asking questions about the purchased curriculum. It is common for other types of support to end after a membership has ended, but I still encourage you to find a program that offers at the minimum, email support for a lifetime.