Another such example is SELinux, which would have prevented so many vulnerabilities from being exploited, but whose poor UX also caused everyone to disable it at install time.
SELinux's UX was significantly improved many years later, but already too late to change ingrained opinions. There are a lot of ingrained opinions about IPv6 too.
IPv6-only ISPs might hit other issues, though. They have to bridge to IPv4 somewhere.
in what way?
And even for system services, you can disable SELinux for one service (permissive mode) and leave it enabled for the rest.
This has been the case for more than 10 years, but the damage was done. It's now very hard for users even considering learning the basics (which are not hard).