We might consider Bitcoin as a legal tender in the future but I'm in doubt about thinking and considering it as the main currency.
Me too. Not really sure whether BTC can be the main currency or not. I only expect BTC can be used in more countries as a currency (
payment tool). Although in the future people may be more familiar with the internet and prefer buying online, we don't know if BTC can be the main option to pay for all the stuff or services.
even IF given the chance to be considered as THE MAIN Currency, I think it was very expensive than of today.
Why are you so confident the price of BTC can be more expensive once it becomes the main currency?
Remember that it is not about BTC to be the main asset in the world, but to be the main currency. I think there will be a different situation here.