Has a game on steam ever been withdrawn from people who have bought it? I’ve certainly heard of cases where a game gets pulled from the store and people can no longer buy it, but never where already purchased things have been revoked.

In addition there are games on steam that you can just copy to make a backup of it and it will run fine (I know kerbal space program was like this for example). In those cases you have exactly the same amount of control as you would have for owning a disc, but with all the benefits of digital distribution.