> If at least C solutions took advantage of abstract data types as advocated by modular design approaches

People have been writing C code with ADTs and "Modules" from the very beginning.

Two excellent examples which come to mind are; Andrew Tanenbaum's Minix book Operating Systems Design and Implementation and David Hanson's C Interfaces and Implementations: Techniques for Creating Reusable Software.

And of course the Linux Kernel is full of great modular C techniques which one can study.