Anything else would also be completely insane. Usually your two options are Android or yocto depending on how custom you want your system to be and whether you want to rather write an android app.
There is nothing worthy of criticism in the fact they choose android as a base.
A competitor (e.g. Google) could come up with an app based version at any time. That app based version could simply integrate with the apps you have already installed. So far this thing is just a quirky feature phone.
There is nothing worthy of criticism in the fact they choose android as a base.