You greatly overestimate the number of users who care about how closed or how open an ecosystem is.
If the user can't interact with their bank, whatchu YouTube, talk to their friends and parents over Snapchat or Skype, call Uber, play Spotify, etc etc etc, it doesn't matter in the least if your phone is 100% open source.
Microsoft had sunk billions into trying to get the ecosystem off the ground. What makes you think that "a truly open system" will get that ecosystem?
If the user can't interact with their bank, whatchu YouTube, talk to their friends and parents over Snapchat or Skype, call Uber, play Spotify, etc etc etc, it doesn't matter in the least if your phone is 100% open source.
Microsoft had sunk billions into trying to get the ecosystem off the ground. What makes you think that "a truly open system" will get that ecosystem?