50 years isn't all that long - there are plenty of tapes and records that are still perfectly usable from then. As long as you included a decent amount of redundancy you'd be alright with a few hard drives surely? There's always the issue of software being able to read the data, but we have no problems opening images and documents from 27 years ago now. In 50 years time there'll probably be a niche industry producing software that converts old formats - just as there is now converting VHS/Cinefilm.