If a web application such as an open-sourced CMS wishes to do well in the Internet, the people behind the application need to mind a few things so that third-party developers and designers would not have problem with the application in the future.
For example, WordPress is liked a lot by its developers and designers as even though it experiences some upgrades, most of the code remains the same. No significant changes are done to the code. This either makes upgrading their plugin easier or allows it to be compatible with the upgrade.
Magento on the other hand has always been a headache for its developers. There are so many different codes being added with each change of version that plugins that supports a different version will not support the latest version.
In addition to that, the interface shouldn’t be getting harder to use. Instead, it should be easier. Joomla still has a long way to go as its backend can be quite overwhelming for a new user. WordPress on the other hand, is very easy to use and is user friendly.
All in all, open source is still the way to go. There are so many applications today that are made open sourced and a lot of these are a champion in one way or another.

