I predict a pretty strong 'Nope' ;)
but the downfall would be new features and possibly performance.
Unfortunately, those two points are pretty much the entire reason for the current compatibility philosophy. :-) In order to convince the entire development community that this is the best approach, you'll have to explain why this tradeoff is worth it when performance and features *are the reason people upgrade to new versions of Drupal*.
With module backwards compatibility, it would allow users to upgrade to the latest Drupal core as soon as it is released rather then waiting for module developers to upgrade their modules. Sometimes modules never get upgraded and it is a real shame IMO to see all that work goto waste.
It is. On the other hand, if people don't need the new features and performance boost that comes with a new version of Drupal, there's not much reason to upgrade. Security patches, yes -- but maintaining security patches is, according to the philosophy you're advocating, easier than upgrading contrib. --Jeff