I don't see it as being a threat at all. It's just another option in the Coinbase system. Kinda like complaining because some multiwallets support a lot of cryptocurrencies and maybe some they shouldn't. It doesn't mean a thing if people just don't use it.
You are right. I' support and I will add that ethereum will complement bitcoins. They are not competing and they are both cryptocurrencies that can be used online. But there is no need to fear because even though many cryptocurrency are existing it will increase bitcoin users. At the end of the day they will exchange their other coins to bitcoins.