http://wordpress.org/news/2010/07/eol-for-php4-and-mysql4/
It doesn't need a rewrite. Instead, work with the many brilliant devs who have been improving it for years and know it inside-and-out -- including its flaws and areas for improvement.
The only reason they are abandoning PHP4 because it has become so rare they can't test against it anymore, not because they want to improve anything. Remember this crap: http://ma.tt/2007/07/on-php/ ?
Which is typical for WordPress: one of the most successful companies to be built on PHP, and all they do is bitch about it and use their influence on a part of the community to hold it back instead of contributing, like Facebook, Digg and such.
Source: http://wordpress.org/extend/
I'd call the API a success: it has thousands of authors releasing thousands of plugins and themes and this is only the code that's been released open-source.
Just because it doesn't look like how you envisage it should doesn't mean it doesn't exist.
Matt's post was true at the time and WordPress.org the open source project has never been "holding back the php community". Instead we have been ensuring that the most important people, our users, are able to upgrade reliably and easily on whatever hosting platform they use.
In the end we put the users first because without them the project would just wither away!