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- Alts mostly need bitcoin to trade and some bitcoins might be always reserved / locked for this.
This one is obviously more about alts needing btc, not the opposite.
- Alts might help to generate a closed bitcoin economy
How comes? Maybe you are right but I don't get it. I thought that alternative currencies make bitcoin more open and exchangeable for something close to it and so they all become parts of one economy.
Anyway, I think it is true that bitcoin won't be alone forever, because when there is something strong it got to have an analogous strong opponent, like USD and EURO, good and bad etc.