I saw the RFC for IPv8 recently

nitpick, but I would say "an RFC", as there's been a number of these over the years

you've gotten a couple responses so far, but I think the central issue is that "complexity" isn't the problem with IPv6 (and one could certainly argue that IPv6 is actually simpler)--the problem is compatibility. This article lays out the issue very well, and also links to this article (which is a more specific look at the IPv8 proposal you refer to). Both point to the same conclusion, which is that fundamentally--on first principles--existing hardware does not know how to handle the upgrade, which will require some sort of dual-stacking, which is the issue IPv6 currently has. (Not its technical merit.)