We've gotten lots of positive feedback for this decision, both directly and on public forums. There's also been some valid concern from long-time users with libraries of old scenes and a dependence on C.O.F.F.E.E. (pun intended).
Fear not! We're here to help!
MAXON is already putting a major focus on developer support. MAXON's four-person SDK Team provides free developer support for everybody. Regardless if you are a professional plug-in developer or a user looking for advice for Python scripting in a scene, everybody is invited to discuss technical questions with us in the public Plugin Café forums. The SDK Team is constantly working on improving the documentation of our C++ and Python SDKs (Software Development Kit). News for developers and updated documentation are provided in our development blog.
Plug-ins to the Rescue
In order to make the transition to R20 as smooth as possible, we will soon release two plug-ins at MAXON Labs. The first one will provide a means to scan scenes for using C.O.F.F.E.E. and to scan a Cinema 4D installation for user-installed C.O.F.F.E.E. plugins and scripts. While it's usually quite easy to port C.O.F.F.E.E. code to Python, Plugin Café forums will be a good place to discuss any issues you might run into during the porting process.
The second plug-in for Cinema 4D R19 will convert scene files created with versions prior to R12 (or entire directories thereof) to the latest scene format. This will be possible via command line for batch processing as well as from within Cinema 4D.
Never Stand Still
As we are always trying to improve our support experience, we will soon announce the MAXON Registered Developer program. This will be our next step in improving and intensifying our relationship to 3rd-party developers, with the goal of better and faster plug-in releases for future major versions of Cinema 4D. Stay tuned!
Let's modernize and move forward with joy in 2018.
SDK Team Lead 
Andreas Block