if no altcoin, may not be the trading coin that we know today. there may be, but there would be little option to redeem bitcoin with the existing currency.
I also agree that without altcoin you cannot do trading because its the alternative of bitcoin and you can only use alt coin in the trading market. You can convert your altcoins earned to bitcoin and then to the currency you are using in your country, to make a clear definition altcoins is nothing without the existence of bitcoin and do remember you must use altcoins for tradings of crypto currencies.
Without altcoins you will not be able to trade against other cryptocurrencies that is correct, but you could trade any other pair like BTC-USD, BTC-EURO, etc.
That is right. The bitcoin is the base coin or currency in the digital world. So It is better to own it than other coins.