Some of them only but it's easy to spot the legitimate altcoins, you have to make sure you invest in a big funded ICO and altcoins that has a good roadmap.

yeah most altcoins designed for a scam attempt and it will always be like that
no altcoin can compete with bitcoin , it's too big as a pioneer
you cannot say bitcoin will last forever indeed
but based on current condition bitcoin far strong better than any other altcoin.

Without alternative coins bitcoin could not do options like alcoins trading because it is a form that can be used in trading and its so important because in trading there is possible high profitable investments that is always an opportunity specially for those who wants secure investments. Altcoins and bitcoins is more like partners in all situation without the prescence of each other its not complete at all someone is missing if we are talking about bitcoin, and its more useful in any aspects of investing digital currency.

'Nothing can last forever'. Yes you are right. Even though we are confidently saying that bitcoin will be there for long year it doesn't mean that bitcoin will last forever. If the bitcoin supply will be low and the price is very expensive then it would make other investor to hesitate if they will invest in bitcoin so they will find for alternative coin that offers almost the same service just like bitcoin.

Why are you worrying about stuff that is out of your hand?  I don't mind having millions of altcoins being created.  There is always a selection process where scams and not good altcoin will be weed out and the legit and good one will remain.  YOu cannot control other people, sometimes even your relative when it comes to deciding on what they think is good for them.  

I don't think that those investment invested on other cryptocurrency will be invested in Bitcoin if that cyrptocurrency does not exist.  They invest on that alternate currency because they value it more than Bitcoin.  They think it is more important than holding bitcoin.

Well, ICO is not always trustworthy, some of them even turn into real scammers and steal all the money of investors. You need to have a decent knowledge about the blockchain system, altcoin develop in order to find a successful ICO. Yeah, I have seen many projects which only need a small fund but still can beat many other cryptos

ALtcoins are dependent on bitcoins. If you notice on exchangers the only way to purchase or to avail an altcoin is through bitcoin. Why is it? The reason is simple, bitcoin can be exchanged to dollars easily and even to local fiat currencies in many countries while altcoins cannot be. Altcoins still needed to be traded to bitcoins so you can convert it to cash.

On the other hand bitcoin can survive without any altcoins since it is backed up by dollars and not by altcoins.

I think bitcoin without altcoin will continue to run normally, but on the contrary. Altcoin could not walk without Bitcoin. And I think altcoin  is a nice discovery, because with it we be able to have other options if the price of bitcoin was not friendly. hehe

The thing is for us daily traders, as well investors, we are always searching for the next coin that might get better value over time, soo we split our investments into more then one coin, i do see this as a way to minimize the risk. But lets think if all altcoins die, i will have to risk my money only at bitcoin, soo this should result into a bigger change in the price even with the big price we do already have. For other side, each person would not be able to get or make amazing results and get more extra bitcoins without trading, we would see only people with big purchase power getting bitcoin, something its unrealistic nowadays. Altcoins does allow anyone to start from zero and achieve good results, the same way some people does with currencies when they play forex or do make sport bets.