Fixing that network you had an issue with to have working NAT-PMP or whatever is going to be infinitely easier than IPv6ing the entire world.
I can't just ask my cell provider to change their CGNAT setup. There's nothing I can do on my end.
1) To the extent to which you can't ask them to change their CGNAT setup, you also can't "just" ask them to upgrade to IPv6.
2) In fact you at least can ask them to change their CGNAT setup, as they at least could, if they chose, unilaterally fix their CGNAT setup, while--even if you also felt it possible to ask them to upgrade to IPv6--upgrading to IPv6 as the solution requires the rest of the world to upgrade as well.
The real question is then: which is solution to your problem is easier and cheaper to obtain, and the answer is clearly not "upgrade the world to IPv6". "I couldn't convince my ISP to fix their CGNAT setup, so I'd rather convince them--and everyone else--to upgrade to IPv6" makes no sense as a coping mechanism.
Sometimes. I've experienced a few networks where even with STUN I'm still not able to get a workable session in IPv4.