I can only share my experience of LBBB with you. Life expectancy is a bit above my pay grade.
LBBB, on its own, seems to have little negative impact on life expectancy. I was diagnosed with it about 30 years ago. It was mentioned to me at that time that a pacemaker was in my future . That is because LBBB is an electrical problem and a pacemaker will correct it. Some 25 years later, that became the case.
Until that time, however, LBBB did not cause me any concerns. And, by the way, I am 70 now.
As for your diagnosis of idiopathic cardiomyopathy, I have Dilated Cardiomyopathy, with LBBB which has resulted in heart failure. Since I have never had a MI nor do I have any arterial issues, it is considered idiopathic, although possibly caused by a virus. And you mention your EF rate at 35%, which is below the 50 to 70 % range. Have you been diagnosed with heart failure as well?
In 2022, I was given a Cardiac Resynchronization Therapy (CRT) device, complete with a defibrillator. A biventricular pacemaker. Sounds scary but, honestly, it has saved my life. Amazing technology! My EF went from 19% at the time of surgery to over 50% just 6 months later. And it is still there. My quality of life is outstanding because of the device.
I am also on some medications which reduce the work load for my heart - a beta blocker and Entresto. But, by far and away, the main reason I have improved so much is because of the CRT.
Have your physicians indicated any course of treatment? Are they considering any kind of cardiac device? Although it is probably the last thing you want to think about, there are literally millions of people in North America with one of some description. Pacemakers are tried and true technology.
Try not to worry. There are answers for your condition. Relatively simple ones. You have a long, full life ahead of you.
Best wishes.