There is possibilities that altcoin can challenge bitcoin but it takes a long time and need a stable and high demand in the market to surpass bitcoin,because bitcoin is the pioneer of all the crypto currency that's why bitcoin has a high popularity and high demand in the market,there is many potential altcoins in the market that it's price pumping day by day and has a low transaction fees but altcoins need a big support with bitcoin because altcoin cannot easy to replace bitcoin.