About that BNB, why should it have a use case outside the platform? Until people code dapps for that bep2 platform it's just coin for decentralized exchange.
I don't understand your point on POS, it's just another way of mining and dilutes the supply same way as POW, how does it effect to price comes back to the supply and how fast does it rise.
Having other use case than being an exchange utility token is always a good thing. If BNB enable its native token to be used outside BNB, then its market dominance will surely expand. More usability means more demand.