The Super 70's weren't built straight from the factory, but rather they were -60's that were re-engined. There have been a number of requests for re-engining, but they were shot down over the years. It was heavily requested when the hushkit "upgrades" were happening but I believe there were a couple issues with it, one of which being that the technical model for upgrades only allowed one per aircraft, so one could not hushkit AND upgrade engines. I think putting CFMs on 707s and DC8s could be a very interesting dynamic/business case for keeping them around in game longer.
However, due to the "too small" penalty kicking in fairly early on, it's unlikely that re-engining them will improve their later years usage unless that too is modified....