Obviously many people will use open source without contributing. The point of making it open source being that others will run into bugs, let you know about them, and maybe even try to fix them, especially if they also become or are reliant on the software. They might even want improvements to make it work for them and again work on them if they have a need for it. If a software is good and/or mature enough it doesn't need to be open source in the first place and doesn't need contributions. Wordpress has sort of reached that mature place where it is already 'good enough' but you can't now un-open source it.