There is nothing obsolete in web development, nor in Ruby on Rails. Never before web development was so popular.
Having support for mobiles is sweet, but you don't have to. It also depends on your application.
If you're doing something which is intuitively imagined in the context of a mobile device, go with mobile devices (I hardly imagine a GPS for desktop computers).
If you're doing something which requires large screens and no mobility, then web (or desktop) application is fine (I hardly imagine an iPhone version of Photoshop).
What is more disturbing is the beginning of your second sentence. Are you intended to invest yourself and your money in a startup company around an application which is written in a language you're currently learning? I would rather take a risk with a language or a technology I know and practice for the last five years. Are you really ready to take this step? Do you have a solid business plan? Do you know highly professional web designers you will hire? Do you have enough expertise to ensure that the web application will be solid, stable, free of bugs, maintainable, well refactored and secure?