In general, I think it would be a good idea to provide some statistics of how many packages would break as the result of a backwards incompatible change.
Agreed. And it should be required as part of release processes for GHC.
One possible alternative, or complementary, action would be automating some kind of pool or voting from hackage packages maintainers. This would give a more informative answer than just breaks/doesn't break. The example that startted this thread would certainly break some packages, but I believe 99% of maintainers would say they could fix their code for compatibility before a new compiler release. (Even without voting, an automatic e-mail about language changes could be sent to maintainers, as many may even not beeing doing Haskell anymore. That could include some link to easily remove their e-mail from any package maintenance.) Maurício