Well, let's see, currently the market cap of the following are:
BTC = 5.4 billion USD
The next 500 alt-coins combined = 581 million USD
You may have a point. (I'm not being sarcastic.)
However, the average market cap for each individual alt-coin is very low compared to bitcoin.
It has been hypothesized that the market cap of the altcoins takes away from the market cap from bitcoin. Therefore the ~580 million market cap from altcoins lowers the market cap of bitcoin by an equal amount and creating an additional altcoin would only take away from the total max market cap of all altcoins