It's not open source, but as counter-example I would offer Directory Opus: loads of features, totally customizable, extendable, complete documentation, while so

It's not open source, but as counter-example I would offer Directory Opus: loads of features, totally customizable, extendable, complete documentation, while somehow managing to not get bloated (they've been at it for decades, after all). Old Opera was also very good in the same way, and long before Firefox even showed up.