Everything in moderation? Arguing that a little bit of X is inherently bad because a whole bunch of it would be awful is silly. You can die of water poisoning. Not everything is a slippery slope.
I like a certain amount of convenience and a certain amount of freedom. iOS affords me a lot of convenience and a lot of freedom, including the freedom to stop using it when something better, or just as good but cheaper, comes along. (Consider that my sunken cost into iOS software is probably about $200 whereas my sunken cost for Windows software was closer to $10,000.)
Stallman advocates a world where, for example, I am not free to sell software I write or install proprietary software on my computer. Admittedly he is probably exaggerating for effect, or his supporters assume and hope he is.