I'm not sure about Steam, but there are plenty of horror stories on the internet about "bought X on Y platform and then Y revoked account". The majority of users may never experience this, but the fact is that when you use a service like this you're at the mercy of the service provider, even for basic things like maintaining access to your account.
What I'd like to see is some sort of movement towards "if it looks like a purchase, then it is a purchase". That is, if I pay a one-time up-front free for access to something (even with DRM), we should pass legislation to make it behave like a purchase and limit the shenanigans that providers can pull. (Granted this is a very ill-formed idea and there are lots of details to work out in order to implement it.)