Re: I had not heard of Tizen before
Full Android is too heavy for "feature" phones and getting increasingly bloated for even entry-level handsets. Google's Mobile Services ODM agreement prevents them from using AOSP for those. In addition, Google's become much more prescriptive about Android's look/behaviour, diluting Samsung's look'n'feel and attempts to build their own services infrastructure.
Tizen's trying to pull off the consortium development model of Symbian, but from that I can tell it's only seen real action in Samsung wearables, TVs and a handful of phones.
I strongly believe we need competition for Android, but it's a long and tiring road, as Jolla with SailfishOS also discovered.