Or even LGPL, what copy-left libraries should be using.

Don't get me wrong, the GPL isn't the "worst thing ever" or a "virus", but I feel it's best suited to full applications rather than libraries.

We need more copyleft not less. I'm glad it is GPL and also plan to use it should I need immutable data structures.

Certainly copyleft has a place, but changing the license to LGPLv3 would allow this very useful library to be used much more broadly while continuing to require that improvements to the shared code be made public. If glibc were GPLv3 rather than LGPLv3 (same goes for Boost, etc.) almost no one would use it. IMO this library will either be re-written under a less restrictive license by some other author (wasting time and effort of the community) or migrate to a more open license (LGPL, ASLv2, MPLv2, MIT, etc.).

For something as fundamental as a container library, GPL will basically mean it's not used widely. There's a reason many language ecosystems generally use BSD/MIT licences for almost all libraries (haskell being just one example I know this to be true for). Applications may be another story.