I do not think that this will happen, the bitcoin is way too big and I do not think it can be replaced anymore by another cryptocurrency.
you are right. bitcoin is currently the highest existing coins and other coin I think was trying to imitate bitcoin. I think that the day would never arrive, because most of the major investors, investing in bitcoin, and it will make bitcoin prices continue to increase. if there altcoin emerging, and has a high price, then the price of bitcoin will also rise.
So much imitation that the only sole purposes is to gain profits out of those investors.
Look at some coins (but not all) only care for the price but doesn't care for the technology unlike in bitcoin that people can benefit even it's used as currency or investment.