So in the next cabal-install release (which should be pretty soon now) configure will do the same thing and pick base 3 unless you specify build-depends base >= 4.
... and so there will never be any incentive for these many packages to migrate to base-4, which also has consequences for packages that do want to use base-4, but also want to depend on such packages. And so base-3 will live on in eternity, and there was never any point in doing that new base release at all. I really really think this is the wrong way to go. Occasional destruction is desperately needed for progress, else things will invariably stagnate. I would suggest as a less stagnating approach to issue a warning/hint when a package with no explicit version dependency for base fails to build. The hint could suggest to the user trying to build the package that they can use 'cabal instal --package=base-3'. Cheers, /Niklas