I could see them coexisting to be honest. They both have their own advantages and uses.
I agree with the idea of the co-existing instead of one replacing the other. STREAMR is another interesting concept, but not offering the same solutions as IOTA.
A token built on ethereum network cannot possibly be compared equally to a separate coin on TANGL network.
I'm still reading up on STREAMR. Feel free to enlighten me, we are all learning a little more each day