While I suppose you COULD learn Chinese only with Pinyin, I can't imagine how painful that would be, and you'd never be able to read anything or be considered remotely literate. Probably why they never changed to pinyin.
Anyway, most of the comments here seem to be related learning to read, which in Chinese is a much easier task than writing. I can read newspapers just fine, but I gave up trying to memorize exactly how to write each character a long time ago, and just use pinyin input on my computer to write when I need to. I feel sorry for students who have to write with their hands, it really is quite difficult and slow for non-native learners. Even my Chinese friends can't remember how to write many of the characters they can easily read, and often if two or more them are in the same room when I ask them how to write a character, they will argue for a few minutes before consulting some reference to figure out exactly what the strokes are. They're also very used to computers doing that work for them.