Yes, you have to choose your dependencies carefully, but where does it end? Steam could go away tomorrow. The game engine you license could go out of business. Your hosting provider, your DNS provider could kick you off.
If you don’t entertain new platforms, you are leaving money on the table. I’d wager that Japanese game manufacturers missed hundreds of millions of dollars by being late to embrace Steam.
Where does it end? They call out these options as presumably being "safe enough":
- A website on a domain you own - A mailing list - Own and license your Intellectual Property - Sell your merch on your site - Your own reputation
We can extend this logic to an unrealistic degree and do nothing but make your game/app/service completely agnostic to any platform in every way, but we can acknowledge that such perfection is unattainable without throwing out the value of being platform-agnostic in realistic ways.