Rectifier :
Why you wouldn't worry about it? Will GTX 1070 be better than R9 490 in DX12 games?
to answer your second question, we wont know until both cards are released and when dx 12 is more widely used and considered mainstream, right now not many titles are being built from the ground up with dx12, their all pretty much ports.
first question, because its going to be some time before (as stated above) dx 12 is more widely used and becoming the standard. by then there should be new cards out by both manufacturers that will outdo the current cards for cheaper anyways.