Discipline is more effective than motivation as a general means to achieving goals.
That isn’t to say motivation is useless. Motivation is effective as a means to accomplish some goals; those to which you’re already aligned. Someone who loves reading doesn’t need to discipline in order to read. However, this won’t be the case for all of their goals – even those within the space of reading. Perhaps they love reading fiction and motivation fails as a means to get them to read non-fiction.