It's too late in this generation for another serious competitor to emerge in mobile. Microsoft might be able to muscle its way into double digits but everybody else is just too late to the game now.
Better to focus on what comes after phones & tablets, whatever that is.
There is still a lot of dissatisfaction with mobile, and plenty of room for improvement. The leaders want you to marry their platform at the expense of services offered by competitors. A newcomer might make inroads just by making it easy to use various competing services out of the box.
Google Sync was EOL'd 2 days before I bought my Windows 8 phone, leaving me with no way to sync my Google calendar on it. I don't want to be in anybody's walled garden exclusively, I want more interoperability and platform independence. Even Google has plenty of room for improvement.
You act like everybody wants to be the next Apple. I and many others will be happy with a smart phone that runs free software. There's room in this space for "The Red Hat of Phones" to come make some money and foster a loyal, if small, community. I don't know who it will be yet, but I don't think anybody expects Mozilla or Canonical to produce the next iphone.
Did they not say exactly that about Linux in the 90s?
Why do you need a third app ecosystem when a significantly more open phone should be able to run most code written for Linux or the web? Again, nobody is claiming they'll be the next iphone.
If you are Mozilla, then likely it's not better to focus on what comes after. Because you have no idea what that is, and it might not even provide an opening for promoting the open web, to which "app stores" based on proprietary technologies are antithetical.
Better to focus on what comes after phones & tablets, whatever that is.