Altcoins are good for investment too because it is very cheap to start especially for the beginners with a very small capital. If altcoins did not exist you will force to buy bitcoin directly which is very expensive to purchase. Eth is good to pick because most ICO's sponsor it as mode of payment for their campaigns and now secondary to bitcoin.
The fact, if you have $100 for investing into cryptocurrency, you can still choose Bitcoin for investment, Bitcoin price is not the reason to make you cannot use that amount to buy Bitcoin, because you can buy Bitcoin with any amount, as long as that amount has same value your funds. 
. Do you forget Bitcoin have unit Satoshi for the small amount?