I'm guessing they all (the linux distros), in one way or another, add value given each project's mission statement. Or at least they believe they are. Otherwise they probably wouldn't be doing it.

And I don't think we'd have to fear the OS will become a "glorified browser shell" if it wasn't the primary "app development platform".

Right now native app development is feasible. You have pretty much 3 platforms you really need to worry about developing for in order to have access to the largest amount of market share for the least amount of effort. Imagine even one more player enters the market and gains any respectable amount of market share.

Instantly every mobile app development team's efforts need to grow or every developer on each team's responsibilities grow linearly with the total number of platforms they need to develop for. From an operational standpoint it's not sound economics to make business decisions based on approaches that don't scale well.