Where is google+ mentioned in my argument? Its not. However, for sake of argument, if g+ proves to not get enough users eventually either, I'm sure they'll shut it down too and try something new. Just like they did with wave.
It isn't in your argument, but is the elephant in the room. If low usage was really the cause for GReader sunset, G+ should have been chopped too. It wasn't, hence it isn't.
Every conclusion derived from here is then flawed.