Clearly the 2017 is like the previous 650's, it's either full exhaust of nothing.
That being said, the deal with the 650 is more about where you want to go with your riding. If you're thinking along the lines of a cruiser, but don't want the shame that goes with one, then this is the bike for you. If you're coming off the 300 and want something to further develop you're skills then stay away from the 650. If you've fully explored the potential of the 300 then there's nothing the 650 has to offer. (Trust me, I have a 650.) Chances are though you've not explored the potential of the 300.
Your argument is kind of pointless in real world riding.
If you want to be a Moto GP racer, sure, you should be able to ride a Ninja 300 at full potential before even thinking about moving up.
I don't know about you, but for vast majority of riders out there, there are benefit of moving up even though they have not explored the Ninja 300's full potential on the race track or something.
It's the sound, acceleration, flickability, braking capabilities, stability, etc...you can definitely feel the difference between 300 and larger displacement bikes. You don't have to be a pro rider to feel the difference between Ninja 300, 650 and ZX6R.
Some people say getting a Ferrari is pointless if you haven't pushed what Honda Civic can do at its absolute limit, but you can still feel the difference in acceleration, handling, sound, ergonomics, etc.