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.
Would you keep your BTC or sell it for fiat :p
Anyway the last Bitcoin bubble was also a boom for alts, the fact is there is alot of people who missed the BTC train or like to gamble with their wealth and alts are a large secondary market for those hoping to make big money. The good thing is with alot less shitcoins now being pumped out, the good alts should hopefully rise WITH the next Bitcoin bubble.