Yes you kind of “own” your ebook, but… The company that sold it to you can remove it legally form their listings, taking away your ability to download your book.
That’s not what I’m referring to. If you have an epub or similar file, you own it. You can store it, and delete it at your own whim.
Yes, but computers are prone to failure, you are depending on a device to hold all your media. I’ve had backups and backups of backups, still losing things to failure. My books take up space, but they will be there unless my home burns down.