It's really a pipe-dream, considering how deeply ingrained the present architecture is in the very core of Drupal.
But looking a bit more closely, there already is a kind of pseudo-object-orientation in the architecture: Function names are prefixed with category names (module name, theme name, "drupal_"), similar to static member functions of a class. Nodes are handled as objects already, it seems.
So the only real disadvantages (apart from changing all the code) would be:
- Permanent deprecation of PHP 4 support
- A lot of changes to the API and required updates to all modules. But we've come to expect that with new major releases...
Whereas the object-orientation would make it a lot easier for modules to interact, less easy for modules to cause function name collisions by forgetting or misspelling a prefix, and easier to draft UML diagrams and that sort of thing.
So, still a pipe-dream. Probably not in Drupal 7 or 8... but maybe 9?