It's not the same coin and the supply doesn't 'magically' add up together. It just doesn't work that way. If people agree that Bitcoin - and only Bitcoin - is worth something, it will be worth something just because of that. Sure, altcoins siphon off a small fraction, but that doesn't change Bitcoins first mover advantage which is most important in this case!
Agreed. There is a limited supply though, and you can't change that.
False.