…and now a growing number of Haskell users have made the judgement that the “upgrade cost” is now too high, otherwise they probably wouldn’t still be using version 8.x.y now - if it’s not being used and it’s expensive, is it really there? Furthermore, there’s already been a variety of new features which have been transferred to older GHC versions.
However if there was another Haskell compiler, that process could go in both directions: patches which mend problems in the other compiler could also prove useful in GHC.