Hi!
According to PSR-4 http://www.php-fig.org/psr/psr-4/
The fully qualified class name MUST have a top-level namespace name, also known as a "vendor namespace".
Does this imply all classes in our code base must start with \Mediawiki? If so, I don't think this is a very good idea. This is a good idea for libraries, especially ones that have good functional unity (i.e. largely do one thing) or part of one big framework (e.g. Zend Framework or Symphony). For large and mostly stand-alone apps like Mediawiki I wonder is there's much point to have a thousand classes all prefixed Mediawiki, given that mostly everything we deal with is Mediawiki, and what is not, already has its own namespace anyway.
If we have a library that is a) related to Mediawiki in substantial way and b) reusable outside, then Mediawiki prefix would be appropriate. But not sure whether it makes much sense for extensions...