You're using mobile-first and mobile-only as if they are interchangeable terms. They’re not.

Mobile-only is pretty much Snapchat’s strategy (and others’ before them): providing an experience on mobile only, because that’s where your users will take most value out of it — and maybe considering other platforms at some point in the future. There’s a merit in discussing if mobile-only is a valid strategy for every company; all in all it’s a good discussion to be had in our ever changing landscape.

Mobile-first on the other hand, has nothing to do with this. Mobile-first web design is a method for designing websites that aims to start from the smallest possible experience to build up components that will later on be more easily adaptable to bigger screens.

It’s not a matter of doing only mobile, or even launching the mobile version before the other ones — it’s about building a mobile version first, because given the current state of our tools, that’s what makes the most sense.

Mobile-first web design does not care about iOS vs. Android, neither about tablets vs. smartphones. Screen sizes are hardly ever considered nowadays, to be frank: you pretty much define the biggest and smallest screen sizes your users will need support for, and everything should work smoothly all the way between them. I don’t care if my users have 400 different screen-sizes; we have the tools at our disposal today to not have to worry about that.

Your comment implies that mobile-first means designing multiple versions of the same layout, which couldn’t be further from what the actual process looks like.