http://coinmarketcap.com/
There is not 1 dollar invested in Alt coins that would not be invested in bitcoin were it not for the competition.
That should tell you something.
The Slow Growth of Alt Coins = The Slow Death of Bitcoin - or at least, it's proportional to a degree.
There is SO MUCH MORE money to be made in the Alt coin explosion.
Altcoins induced more Bitcoin buying, since people use mainly bit coins to invest in alt coins and increase their bitcoin holding. People take their altcoin profits in bitcoin not USD.
So on the contrary, ALtcoins help keep bitcoin's price high infact i believe we never saw a massive crash due to bitcoin buying for altcoin investment.