Backward compatibility always trumps new features.
DAVID CHISNALL, UNIVERSITY OF CAMBRIDGE
On my way out of academia, before Cambridge persuaded me to return, the last paper that I wrote was a description of a new Objective-C runtime for use by the Étoilé project.1 An Objective-C implementation requires two components: a runtime library that implements the dynamic parts of the language and a compiler that emits calls to this library.
http://queue.acm.org/detail.cfm?id=2331170
Related:
A co-Relational Model of Data for Large Shared Data Banks