fat16lib:
This won't happen as long as the Arduino company controls the IDE and you fit your stuff into that environment. The average user just won't use Cosa since what they get from Arduino is good enough for hobby use.I was involved with BSD Unix at Berkeley. The goal of BSD was to replace AT&T Unix, not fit in. The open source Unix evolved into Linux and is the base for OS X and Android.
Open source Uinx happened because AT&T was impossible to deal with. The Arduino company provides a product hobbyists like or at least live with.
I have ported popular open source RTOSs to Arduino but don't see much interest.
I just accept Arduino and build little add-on libraries that may be useful to users.
Supporting a movement like Arduino is not an easy task as the main focus is not programming and the majority of users (and founders) are not programmers. But all this risks graining to a halt when scale starts to play in. The lack of structure will need to be addressed sooner or later and the foundation needs to be production quality; rock solid.
Using a Due with current Ardunio software should be compared to, for instance, using a RaspberryPI. Due gives for the average user just more memory, speed and ports. Trying to integrate different libraries is near impossible as the problem was not so much the lack of memory as the lack of structure. I think that the Mega has already shown this problem.
Professionals don't really use the Arduino IDE. It is basically a toy. A nice toy - but still a toy. The thing is to work around it and replace it in increments. Show the alternatives - slowly. Obviously Cosa can be built without any of the Arduino IDE/library/etc. The only essential part is AVR tools with GCC etc. GNU emacs and make works just fine as an IDE replacement 
I think it is important to show possible steps forward with regard to structuring and using the full power of the programming language C++ for small scale embedded systems. I intend to include a Forth-like virtual machine (working name Avanti) in Cosa. This would gives direct access to the Arduino hardware without a compiler when needed and the possibility to add Domain Specific Languages for teaching, rapid prototyping, etc. The concurrency.cc project is a good inspiration in this regard.
If you think OOP is too larger step for the Arduino community then RTOS is "from here to eternity"
BW great comments and insights!
Cheers.