Simon PJ writes:
But if you say ghc -c Main.hs -package base -package haskell98 then the (implicit) import of 'Prelude' will say "Ambiguous module name: Prelude", because it's exported by both 'base' and 'haskell98'. [...] So: Under Plan A, some Hackage packages will become un-compilable, and will require source code changes to fix them. I do not have any idea how many Hackage packages would fail in this way.
At a crude count, there are 368 packages on hackage that depend on both base and haskell98, and thus will be compiled with a fatal command line like the above. Of these, somewhere between 105 and 199 currently don't build (uncertain because I've only tried to build libraries). So this change will probably break around 200 packages. On a different point, whatever you do with Prelude you'll probably also want to do with Foreign and Numeric.