I think I know where you are coming from here. This kind of news can cause a lot of worrying thoughts, I've been there as have many others on this forum. Some of us still are with blockages still existing, yet to be addressed.
In the medical world, a 70% or higher blockage is considered the very high risk and should really be intervened if possible. Anything below this is a risk, but not as extreme.
It is not really possible to put a likelihood of rupture on your blockage because many people go through their entire natural life with it never rupturing. However, there is the strong possibility that it will grow. This is something you might want to address with your Cardiologist. If he puts you on medication to mask your symptoms, then you may not realise the blockage growing and becoming extreme risk. What if you don't notice any stronger symptoms until it reaches 90%? Good questions for your Cardiologist.
If he says he will wait until you have stronger syptoms and you happen to feel strong angina when it reaches 70% or more, what will he do then?
Of course, it may never increase. If you keep your blood pressure in the normal range and don't get stressed too often, then you will lower the risk of a rupture even more.
I assume you are on asprin? this will help prevent the second clot forming (the killer) should a rupture occur.
One Cardiologist offered me a treatment of having certain cells associated with Angina destroyed by laser. When I asked how I'm supposed to know if I am damaging my heart during exercise or whether my condition is getting worse if I feel nothing, he suddenly shook his head and changed his mind. He replied saying that the heart would not be damaged. I then asked for him to put in writing that lack of Oxygen to heart tissue will not cause damage and he simply shook his head and changed his mind, saying it wouldn't be ideal in my case. Asking the right questions is sometimes important.