Mobile first?

Is this a good idea, or should it only be done in the event that the app being created is especially suited to mobile use?

I can see mobile being a great (and convenient) extension of an already existing web platform, but I can't see it by itself. I'm thinking about creating an app, but what I'm about to do would be just a regular web app with no mobile extension. Is this a good idea nowadays? I'm thinking this way because I don't use mobile very often (read: not at all), and only see it as a means to text and take pictures. Am I going to be missing out on something by not building a mobile app?

Also, it seems like the mobile app market is saturated. Is it even possible to etch out a space in that ecosystem anymore as a newcomer?