I think it will be the legality issue that will be a problem first but would not kill bitcoin, it will still go on. The thing about the new development of a new coin that will surpass bitcoin will take more years. We know it is too good now and cant be replaced just like that, add up the high value of it now.
Back to legality, almost all countries wont accept it and others just regulate. How many years have bitcoin been here and yet not a lot changed regarding countries accepting it.