In 4.0, we will remove the old compiler and then we can see how to make the API perfect.
Will it be able to decompile by then?
I believe we cannot remove the old compiler if we do not provide something to access the method's sources.
It will have either a decompiler or in-image sources if we achieve to compress them enough (recently a guy compressed the AST in 15Mb by sharing nodes, which is half the size of the sources).