i wonder: if Bitcoin will become the main coin, what will happen to the money we are using now like us dollar?
You forget that the biggest driver behind fiat money is the government. As long as taxes are due to the government and they have to be paid in fiat, there will be use for your dollars and euros. No government is going to accept that bitcoins are more valuable than the fiat they print; hence the government will continue to accept fiat money at least for taxes and penalties.
I think that it may affect our economy because if bitcoin peso will be our main coin what will happen to the main peso that we have industrial part of economy will affected, goverment, and people will change and even though your country we must think first before that happen we never know how it will change someone's life and economy.