Long Term Effectiveness of Asthma Medicine

I wonder if any of you have seen your medicine lose its effectiveness over time? My son had been on Dulera for over 17 years. The last year or two he has complained of breathing troubles and has opted to nebulizer treatments in an almost daily basis. He claims it’s because of his pneumonia years ago, but as a 67 year old (me) with asthma, lots of lung scarring and bronchiectisis, I don’t buy that.