PostgreSQL has spent a lot of effort on the extension framework ("create extension") to make it easier to support more external modules. If there is a particular area that needs improving, please elaborate.
Making external modules a normal part of the ecosystem, like with ruby (and other popular languages), is the way forward for postgres and has some major advantages.
For instance, given that PostGIS 2.1.0 was just released today, it would make it into 9.4 (as 9.3 is already feature frozen), and 9.4 will be released in about a year. Because it's an extension, you can install it in 9.2 today, or 9.3 when it comes out in a month or so.
Also, just plain modularity, division of responsibilities, allowing for different licensing, and lowering the barrier for contribution are huge wins. The same reasons that ruby has gems.
So, hopefully we can address the concerns with extensions rather than build PostGIS into core.