Apple has deleted paid music from my itunes account, despite it still being for sale, and refused to restore it when asked. They said the publisher removed that specific version of the album from the store (the new version has an identical track list) so their hands were tied, and to use the file backups I made of those files.
Microsoft threw away my e-book collection when they shuttered their first ebook store back in the day, with no (legal) ability to carry those books onto a new device, even if I copied the files.
So far the only long term reliable digital goods business I know has been amazon with the kindle, but I'm sure you can find horror stories there as well.
You don't buy this stuff, you rent it indefinitely.