But the came I am making is to PHBs, not engineers and I am trying to
find a path of least resistance.
IPv6 is, on average, 20ms faster than IPv4. I don't know why, I just know that the evidence is diverse and compelling that it's true. Fast – Retevia
A faster web site means people find it earlier in Google search, stay on it longer, and buy more stuff from it. IPv6 for Sales and SEO – Retevia
If you're an ISP, it would be nice to give your customers that extra speed.
IPv6 in your data center also means your security team has an easier time tracking down miscreants than if they were behind CGN. Any security tool without IPv6 is blind to 54% of US traffic, 24% of CA traffic, 27% of global traffic.
Renumbering into IPv6 might mean you can make addresses available for sale, and prices are approaching the point where that makes sense. Address Pricing: 2019 and Beyond – Retevia
For ISPs, you should absolutely figure out your IPv4 run rate, i.e., when you'll run out of IPv4 addresses. Then the PHBs have to decide what to do about that: deploy IPv6 and hope it's a viable alternative (with translation?), buy IPv4 addresses (at today's prices or tomorrow's, and how many addresses?), or deploy NAT44 and hope customers are okay with it.
For ISPs, consider how many of your customers are medium to large companies. These customers may need IPv6, either to sell their own addresses, or to connect with branches or partners who are out of IPv4. There are ISPs in the world who only support native IPv4 because some of their customers can't get approval for IPv4 from US corporate HQ. Of course, they pay more for that. For that matter, consider how much you charge for additional IPv4 addresses, and the rate at which customers could decline that service.
(But wait, you say, PHBs don't want to lose the IPv4 revenue! Depends on whether the competition is likely to offer the cheaper alternative)
Finally, the Rabobank argument: Maybe there aren't important sites, tools, or architectures that are only available over IPv6 right now. When will there be? Five years? Ten? (Seven? IPv6 Growth – Retevia) How long will it take you to be completely IPv4-independent, and will it be done in time?
So there's an 8-slide deck for you. Good luck with that pitch! I'm interested in what feedback/pushback you get.