> The plug-ins being a mixture of free and proprietary that can't be easily extended to add features

This is not true - while there are paid (“premium”) plugins in the WordPress ecosystem, they are all open source, as they are classified as derivative works of WordPress and hence the GPL virally applies.

There are in fact some plugin resellers who sell bundles of premium plugins under the GPL - it’s frowned upon by the community, but it’s certainly legal. (At least for the copyright; trademarks are a separate story.)

Typically, with paid plugins you’re paying for access to the download system (making updates easy), and for support.

See also, https://wordpress.org/news/2009/07/themes-are-gpl-too/