But would people really try to share / bookmark a page that at that very moment doesn't show the content they want to share?

I think it is a beautiful, simple solution that just works (and also did work perfectly in the past when we didn't have JS)

Yes. If you have ever worked on a site with lots of users, you will find that if there is a way to break something, someone will absolutely do that thing. It might not be super often, but it will definitely happen.