Same with OS and software support.
In the end some will profit from this (even the lazy ones -- "quick, pay us for a new version that enables IPv6 support) and some will lose (they'll find competitors did a better job at IPv6 support).
Non-critical applications where people aren't willing to jump through any hoops... Well, it's not exactly news that sometimes old apps get left behind in technology transitions.
You could also potentially add a specific wrapper for your application e.g. wrapping MySQL connections inside of stunnel allows them to run over IPv6 in version of MySQL built with v6 support, or for simple daemons you can do things like set up socat to proxy between an IPv4 socket and an IPv6 socket.
Almost no software you're likely to be running is incompatible with IPv6, though. Software support for IPv6 is way ahead of actual network implementation.
How sure are you of this ?
Now, ofcourse all the major widely deployed software, be it browsers, web servers, mail servers and clients, remote file systems and similar are already IPv6 ready. What about all the many many millions of custom built systems and applications ? The kinds you will never see, that's hidden behind a corporate wall in use by 10-100 people.
Or all the devices that people have bought - all the wifi routers, IP cameras, network printers, DSL modems and so on ? It doesn't matter how much anyone says that these should have had time to support IPv6. If they arn't, it'll cost someone money to replace them - not all are willing to do that.
What about user inputs that only accept IPv4-formatted addresses? :P
The bigger issue would be connecting to IPv6 only servers from an IPv4 program. The first question is how many IPv4 only programs exist in the wild. If you happen to have one, and cannot update it, then you still have the option of creating an IPv4 to IPv6 proxy for the specific site(s) you need.
Elsewhere, the IP addresses were used as part of a file name on Windows; we replaced all colons in the IPv6 address with '!' to fix that.
There's bound to be lots of old software with similar problems around.
Sometimes software simply has to be upgraded to meet changing circumstances.