It's not that it will be disastrous. It's that it already is disastrous right now.

The functionality of the Internet is severely degraded by NAT, and has been for all of those "at least 15 years". The network can't deliver its most basic function, which is to let any node talk to any other node at will.

As a result, the designs of huge numbers of higher-layer protocols have been warped and complicated, and the whole thing has contributed to the whole maelstrom of extreme commercial centralization pressures, too.

Everybody who wasn't willing to hard cut over to IPv6 should have been forced off of the Internet 20 years ago.