they don't need to compete with the asic rigs to contribute enough hash rate to get paid decent btc. honestly it's a toss up of whatever you want to mine. but as an example, if you use a single 1080 gpu to mine ethereum through a pool or btc itself using nicehash it's a roughly equivalent payout. if you want btc and not eth, you may as well just mine the btc. sure, you aren't mining nearly as much as an asic rig will give you. but you are mining an equivalent value of btc as the eth you would have received using the same gpu.