I think that this change would make otherwise reasonable uses of hGetContents on large files more likely to end with an exception. It seems perfectly reasonable to use hGetContents to stream data from a either a large source or an infinite source (say a pipe or socket or /dev/urandom). Perhaps you're streaming those bytes into another form, who knows. In any case, file descriptors are precious and it would make sense to hClose the file when you're done with it rather than wait and hope that the GC gets to closing your file.

With this change we'd be introducing asynchronous minefields into code that don't need to exist.