The success of the Protestant Reformation was political, and to a much lesser extent doctrinal or theological. If one would point to longevity as proof, one must point to Hinduism, from 1500 BC; the Celtic religion dates back to 1000 BC, Buddhism from 600 BC, and Jainism, about the same time.
When you remove many of the differences in practice, there is remarkable harmony between Protestant denominations about the role of Scripture, faith, and of Christ.
I would disagree; given the different Trinitarian or non Trinitarian understandings amongst the different denominations (subordinationalism and Jesus-only beliefs are rife amongst the various Pentecostals and non denominationals). Free will versus predetermination is still a significant (but waning) issue amongst the non Calvinist and non Calvinist groups as well.