Massimo, I have the highest respect for your judgement and this platform. It's your ecosystem. You can break it if you want to.
But hear me out. As a long-time Arduino library developer, I can tell you it's both frustrating and disheartening to have one's work broken without warning, again and again.
Massimo: You won. This platform is a huge accomplishment. There are existing projects on Duemilanove boards that will be in operation for decades, perhaps long after we are dead. This is goodness. But unless the Arduino team perceives it to be part of its mission to provide a tool chain that will be compatible over that kind of time scale, those systems will become useless the first time a bug is found and an old library needs to be recompiled to fix it.
I would agree library developers have a responsibility to track the core. But it is a fact of life that there are libraries that are not going to be maintained any more and therefore need to be buildable in their present state.
The thing that is particularly egregious about this proposed change is that it not only breaks current libraries, but it also prevents co-existence of future versions with earlier versions of Arduino needed to build older projects.
From my perspective, the team should take more seriously the impact of its decisions to break backwards compatibility. If there is a way to keep the old libraries working, then keep them working, even if it's ugly compared to the shiny new stuff. In my view, this is a responsibility that comes with the success of this project.
If the core team does not agree with this view, it would be helpful to hear.
Thank you for listening. I'd be happy to follow up either here or privately.
Best regards,
-br