Any other coin rather than bitcoin is counted as altcoin, so Ethereum is but if the number of people using ethereum increases then it means that more people are leaving bitcoin or may come back after ethereum gets dumped.
It is not the possibility that if some one will invest in ethereum or any other altcoin then he will put out their money from bitcoin and will leave bitcoin, it is also the possibility that people invest in bitcoin and when they find any other coin then they invest their other money in other coins and remain their invested money in bitcoins.
I know that it is hard to believe that people that joined bitcoin will leave it so easy, but if people move funds to any other coin it means that the bitcoin price will fall down and maybe to $100 as the OP stated.
If panic selling would happen then it would possible to dropped its price to $100 mark. Its really hard to believe that it would happen to bitcoin as we can see the current state of the coin its stable on its price. We will wait what would happen in upcoming months or years to come.
No, I don't think it will fall in $100. If we are to look at the market. It is attaining its stability in around $650-$750. And considering the continuing growing number of bitcoin community, means that the demand in BTC is high enough to cause he pumping of it's price. And when the rumors become true that bitcoins will be accepted as a moneytary payment, the price of it can reach up to $1500 or higher. So, there's no point of panic selling or something like that.