Between system data, music (mostly in .mp3), and apps, I’ve maxed out my 32GB phone. That said, there’s not much I’ve had to decide to remove, so 64 GB is probably good enough for me, and 128 GB should absolutely last a while based on my current phone usage.
However, if the Librem 5 aims for convergence (which I believe is part of the advertised goal), then 500GB seems far less crazy. If you imagine the Librem 5 being an ultra portable, all-in-one phone + desktop computer swap in, on which you want many types of data (multimedia, office suite type documents/spreadsheets/presentations, coding projects, maybe even datasets for various analyses, though that becomes less likely given the computational limitations), 500GB isn’t insane. Your phone starts to become an interactive flash drive (though that isn’t a flattering way of putting it).
I don’t think we’ll be seeing 500GB in the Librem 5 v1 though.