I'm not involved in the marketing field but here are my two ideas.

Create various beautifully designed promotional badges that are easily included on the website like:

https://www.w3.org/html/logo/#badge-builder

Try changing culture, so people would feel proud by putting this badge (small or big) on their sites.

This could be included on WP.org, so people would be aware about this option.

Just for fun have recognition page where sites with the most referrals would be listed (might be a spam trap though).

Widely publicize cases where people got burned by the closed source (lock-in) platforms.