That's pretty much irrelevant. Engine replacement is about increasing range and/or reducing tanker requirement/airframe wear, and reduction of maintenance costs

That's pretty much irrelevant. Engine replacement is about increasing range and/or reducing tanker requirement/airframe wear, and reduction of maintenance costs because existing engines are ancient and their maintenance costs are high. One of the reasons this program has been postponed for so long was retirement of C-141 that left USAF with plenty of still good TF-33s. But now these are starting to run out too.