Iirc, Stephen Seiler had data to show that there is roughly a 2/3 cross-over between cycling and running. So, 3 hours of cycling = 2 hrs of running and vice versa.

Your experiences coaching aligns well with my own personal experience, and that of my good runner friends. Given the high degree of cross-over, the aerobic benefits couple well with the decreased "running load"... Allowing for recovery from the pounding of high volume running, while still maintaing (or even building) some aerobic fitness.

I have a friend who is a very good marathoner... Completing 10-12 per year, at sub-3hr pace as a 58 Yo male. He does 2-3 50 mile rides a week coupled with about 40mpw running. Obviously, the devil is in the details of making that work. But, it certainly, can and does... When done right.

My n=1 experience is that I can substantially cut back on cycling for a run focus with little/no loss in ftp over a 6 week period, if run volume / intensity is increased. I've done that many times with great success. Rotating focus periods of 6-8 weeks is a staple of my triathlon training during the off/early season.