Rolling out an IPv4 only stack right now is debt. IPv6 has finally hit the point where there's no longer an if portion to the question, but rather a when. Every server rolled out in an IPv4 only setup, every service built, etc, is one more potentially service-interrupting area that will have to be changed when port your stack to IPv6. If you're young and small, starting as IPv6 native means that the next few years you'll have an advantage over your competitors because they'll be thinking migrations while you're thinking about growing your business.