Yes that's true. Files are only guaranteed to be available indefinitely if someone has decided the file to be noteworthy enough to pin and their system is online. Ideally, systems like http://filecoin.io/ will be built upon ipfs to enable those that care enough about their files to obtain distribution through paying a little money to incentivize others to host their files.
Won't this mean that rarely used files aren't really permanent ,which is a known problem with bittorent ?