red hair is caused by a recessive variant of a gene, which means you need two copies of it to be a redhead. Currently, redhead alleles are found at a much higher concentration in northern and western Europe. it's possible that as those genes spread out the probability of two people with a redhead allele having a child will diminish, which might make redheads less common, but as long as the genes are there, we will still have redheads.