I agree with your Doctor. If your bypass vessel has collapsed, then there is nothing which will come from this to cause a heart attack. For a heart attack you need a clot. For a clot you need damage or ruptured plaque.
Approximately 10% of veins that are used in cardiac bypass fail the first year, and 10% fail every year from time of the surgery. No one knows why they fail, but they do.
I doubt your doctor is afraid of litigation as vessel failure is a documented fact. He/she is trying drug therapy to develop what are called collateral arteries, which are tiny vessels that can and do enlarge to take the place of the occluded artery in many if not most cases.
I've just been through exactly what you have experienced, except all my bypasses failed, and I have gone through drug therapy. I didn't develop collaterals, so a new stenting procedure was used to try and open my native arteries, which I'm thrilled to say was a success.
Very possibly the bypass failed as the native artery continued to be the dominant vessel, not the bypass vein. When that happens, blood tends to clot in the vein and it fails.
I'd suggest to approach your treatment with an open mind and have a discussion with your Cardiologist. If you are overweight, I'd suggest a diet. If you are diabetic, I'd work to control it, and follow a heart healthy diet without any cheating. Throw away your salt shaker, and don't eat processed food. Above all, you must exercise. A sedentary lifestyle simply is a guarantee of heart trouble.
Hope that helps, keep us informed.