I get that there are people in the world with severe lack of IPv4 addresses and this is a valid reason for them to switch to IPv6.
However, assuming that I and my organization have enough IPv4 addresses (without requiring any of the tricks of multiple layers of NAT), is there a sufficient reason for us to justify the effort/expense of changing what works?
_You_ might not need to deal with the tricks of NAT, but more and more, your customers _do_.
For example, I'm in the US, and my mobile device has an IPv4 IP sitting behind the mobile provider's CG-NAT. My mobile device also gets IPv6.
Since you only provide your site over IPv4, that means my opinion of your site is governed in part by my ISP's CG-NAT, which you do not control. If the CG-NAT is overloaded, or otherwise having issues, that will manifest to me as problems with your site. I will assume the problem is with your site, and take my business elsewhere. In this example, if your site was also provided over IPv6, then my phone would have skipped the IPv4-only CG-NAT, and things would be fine.
I disagree, because as of now the vast majority of top sites are IPv4-only (see e.g. http://www.delong.com/ipv6_alexa500.html for semi-fresh stats) so if someone gets bad experience on my IPv4 site, they will not assume that the problem is with my site, as that will match their overall experience with most sites on the web.
Perhaps you will diagnose the issue as caused both by the combination of your ISP flaws and my choice to be IPv4, however, the vast majority of users definitely won't, they will notice that most of the web (i.e. IPv4 web) sucks for them but doesn't suck for other ISPs and will put on the pressure on your ISP to get their stuff working to properly support the IPv4 net (or take their business to another ISP) instead of putting the pressure on IPv4 sites to switch to IPv6 or taking their business to another site.
IPv4-only still has enough critical mass (and will have for quite a few years still) to ensure that 100% of ISPs in the world have to support decent service for IPv4 only servers; IPv6-only does not yet have enough critical mass to ensure that all or even half of servers have to support decent service for IPv6-only clients - this is a quite literal chicken-and-egg problem for the motivation to switch.
I'm a little concerned about that site: For its test connecting to Google, besides using port 80 (instead of 443), it reports "500 Can't connect to google.com:80 (No route to host)". I just tested to Google IPv6 ports 80, and it works fine for me.
Regardless, Google's chart of IPv6 adoption (https://www.google.com/intl/en/ipv6/statistics.html#tab=ipv6...) continues to trend towards IPv6 adoption. I expect that if you looked at a chart of web site IPv6 adoption, it would also be trending upwards (though not at as steep a rate).
If that's true, then it stops being a chicken-and-egg problem, and turns in to a game of chicken: Who starts supporting IPv6 first, you or your competitors?
> However, assuming that I and my organization have enough IPv4 addresses (without requiring any of the tricks of multiple layers of NAT), is there a sufficient reason for us to justify the effort/expense of changing what works?
No, unless you're running servers for users who might have those issues on their side (e.g. anyone accessing your servers from mobile). May as well wait until you can save money by giving up your IPv4 addresses.
However, assuming that I and my organization have enough IPv4 addresses (without requiring any of the tricks of multiple layers of NAT), is there a sufficient reason for us to justify the effort/expense of changing what works?