You can opt out in the beginning, but later it becomes a monopoly.
You can't even buy a game on a DVD anymore without it requiring Steam to install, activate, and play.
So, in theory, if they succeed, it won't be optional anymore.
"Oh then you can just stop playing games!" ... yes, but I would prefer not to. I would prefer a fair exchange of money for an unencumbered product.
Hopefully, if I speak out now about the potential and likely dangers of such a service, then I may hinder its adoption and prevent the further decline of an art form that I hold very dear.