The dual ABI is only implemented in libstdc++, so if you want to use the new ABI in a library then all your dependent libraries will then need to use the new AB

The dual ABI is only implemented in libstdc++, so if you want to use the new ABI in a library then all your dependent libraries will then need to use the new ABI so you are all using the same std::string etc.. From this point of view it's perfectly sensible for a Linux distribution to decide to rebuild everything using the new ABI. You're right that a soname increment would have been one way to do this. But you would still have the problem of transitive dependencies on the old ABI since you can't load two libstdc++ libraries at the same time. In a few months time we'll all forget this was an issue since it will for all intents and purposes (for the end user and developer) be a completely transparent upgrade.