It does effect miners pretty substantially, which is why most of them were on the "bitcoin unlimited" side of the arguement instead of the segwit side. Bitcoin unlimited would have given miners much more power and the right to vote on changing transaction fees at any time. BIP 148 will force miners to either stick to what they are doing now, and risk losing all the bitcoin they have mined when Legacy bitcoin merges back with BIP bitcoin, or upgrade their mining Nodes to BIP Bitcoin nodes. What that does to difficulty and profit is anybody's guess though.