Chill out, I think you did not understand my point.
First off, I would be more inclined to purchase & keep a subscription with robux than with real money.
My second point is that, as a premium user who spends already money into the game, I would prefer to be able to buy a subscription with robux I gained through premium, than having to purchase it for extra bucks.
It is also relevant to say that parents would not allow children to purchase subscriptions like that, restraining the amount of potential buyers.
EDIT: Refer to this post extension