Other than that language itself being incompatible with previous releases there is the added burden of having to maintain a whole ecosystem, not unlike many frameworks and their plug-ins.
Many people will write some module or other and will make it available for others to build on, and then an upgrade will break the module. The module creators have since moved on and are no longer supporting their brainchildren.
Fortunately with open source you actually can fix these problems - most of the time - but there is not always time or opportunity to do so.
Backwards compatibility is what made microsoft a dominant market force, I believe that you mess with it at your peril.