Great question.  I think this is something in the backs of many asthmatic minds.  I hope I can allay some of your concerns.

First off, sorry to hear about your losses. I don't expect this to give you solace, but the incidence of asthma related deaths is something like 0.002%.  And since most people don't die from asthma, when asthma does take a life it can be very frustrating.  I know this first hand because I had a friend die of asthma when I was 15. 

Even with a disease like asthma that has been around for over 5,000 years, scientists still aren't exactly sure why asthma is more severe in some cases and mild in others.  In fact, in some people asthma goes into complete remission at times.  It's possible there could be a genetic reason for this, or it could possibly be environmental -- or both; or something completely unknown.

Thankfully, however, with modern asthma wisdom and new medications, most people with asthma should be able to live a normal active life.  And, likewise, about 90% of asthmatics should also be able to live a normal lifespan.  The key is for doctors to diagnose asthma as soon as it starts and to treat is immediately and aggressively.

One of the best ways to prevent your asthma from troubling you is to make sure you see your doctor at least twice a year (that's the new recommendation), to be on whatever asthma meds it takes to control your asthma (to see which asthma meds are best for you click here), be compliant with your medicines, and to have an Asthma Action Plan so you know exactly what to do in case you do get short of breath.

I hope this answers your question.  If you have more questions feel free to ask.