Those most likely are not running against the internet as a whole or at random, or they would be screwed already.
VPNs and IPv4 networks that exist today won't disappear...
Why wouldn't they? The IPv4 addresses would either be behind some sort of NAT, or they would be IPv4 addresses in IPv6. If the apps can't handle IPv6, then they can't handle IPv4 addresses in IPv6, and they definitely won't be able to communicate with newer hosts that have IPv6 addresses outside of the range of IPv4.