Earlier quoted context omitted.
But if you've paid for Amazon Prime then Amazon has become a platform similar your iPhone. Just like you can buy a cheap secondary phone for Steam Link, you can join another company's "free shipping membership program" to buy your Nest (or just pay shipping or get it locally, like you said). Regardless, I think the distinction is less clear than you make out.
Is that a serious suggestion to buy a second phone, or is it supposed to be absurd? How does having prime lock you into Amazon, at all? You're not disallowed to shop elsewhere after signing up for Prime. You don't even have to sign up for someone else's "free shipping membership program" to buy your Nest. You're argument doesn't work for me.
To be clear, I think Apple is shortsighted in this decision and the Steam Link app isn't actually very competitive with native games. But I'm not its target audience either, even though I have a decent library of Steam games (50+).
My guess is that this move is not really about Steam Link but about the next company to come along with something like "play Android games on your iPhone" functionality.