I don't understand why you think that people must buy bitcoin to buy Altcoins... I never bought Bitcoin with fiat money... I always bought Litecoin and Ether... Even for Ripple, you can buy it with fiat money from Bitstamp... doesn't that blow up your theory?
No, different people have different means of getting into the market. It just so happens that a significant chunk tends to enter through other coins which isn't something to be neglected.