what if someday when bitcoin has become the main payment instruments in the world. and how when the Internet network paralyzed. it means that we all can not perform various financial transactions? , is there any solution ?
Use cash.
I guess maybe cash would be dead earlier than the internet.

Cash system never dies. It is the base for entire economy as well the financial functioning in a country. Now the cash system too have interlinked with the internet. So even for successful cash transactions internet is a must, so the internet gets high updation rather than going dead.