Besides all these shiny new things, I'l like to stress that it's extremely important to make sure that a) software currently using swift-corelibs-foundation keeps running and b) software currently using swift-corelibs-foundation can use the new implementations, where they are available.
Since the new Foundation lacks and will probably never get (for me) critical infrastructure, I have to continue using swift-corelibs-foundation for a long time coming.