Good runners won't be bad cyclists. They might not become great cyclists - running doesn't give enough benefit at the faster speeds/higher levels (like Cat1/2 and above) but they won't be bad.

Any runner who's cranking out 5ks in 17minutes or under, or running marathons in 2:45, will blow away almost all recreational cyclists with a mere 1-2 months of training, if not right out of the get-go. Marathon runners are also particularly good with distance - the limiting factor for them is usually butt or back pain, not leg endurance on long rides.

However, it's been shown pretty decisively in triathlon that great runners do not automatically become great cyclists. There are plenty of very fast marathon runners (like 6minute miles for the marathon) who migrate to ironman and expect to dominate but find they're just MOP due to their inability to really crush the bike at a similar level.