Why does it's use actually have to be defined?
You are limiting yourself to thinking within a very small box. Bitcoin has MANY more use cases than playing the role of a payment network "or" a settlement layer. There are limitless capabilities that this network has, some of which haven't even been defined yet.
So to say that bitcoin "must" be either a payment network, or a settlement layer really does put a cap on bitcoin's potential growth
Value transmission is only one small iota of bitcoins capability, we haven't even scratched the surface of what's truly possible with this technology
Because the more complex the protocol is the more risks it carries.
By adding all sorts of junk to it is not a good way when it comes to money.
It must be simple and stable, thats all it needs to be succesful.
If you want complexity then use ethereum or other stuff.