I think Miller is making Hayes "expendable."
Now, that doesn't mean they're going to TRADE Hayes. But, I think now that Miller has finally seen his game come to him lately, in addition to his versatility at C (he's OKAY at it but is good at FOs) and a W (MUCH better there), he's more of a natural fit on this team long term. Assuming Lindberg slowly develops and grows more comfortable as a 3C, Hayes either finds himself priced out, or in a pickle maybe playing a position he's not accustomed to.
Miller is a vital part of this team moving forward. The Rangers currently have five top six forwards: Brassard, Nash, Zucc, Stepan and Kreider (despite his struggles, he still has a boatload of potential and his game is custom made for the postseason). Miller is the F the Rangers are missing to complete that rotation up top. With the emergence of Lindberg, improved play of Etem and the potential that Hayes brings, I think a change is coming soon.
I think Miller will be moved up, Etem will bounce around between the 3rd/4th lines seeing what fits best and Fast will ultimately be moved down. He has earned more minutes. The beauty of this is that if Kreider-Brassard-Zucc continue to gel with more ice time together, Miller fits in rather nicely on a line with Nash and Stepan.