Why sell alts for BTC when they're already priced in BTC? All else being equal any gains you make on BTC are already included.
If Bitcoin went from $250 to $10,000 tomorrow, would you keep your alts or sell all of them for BTC.
I see what you're saying. Last time though alts did really well so even with increased sell pressure from people wanting to cash some alts in to BTC to sell for fiat, the amount of new money coming in to BTC in an event like that can lead some people in to leveraging their positions by getting in to alts.