DaphneMoon.
Yes the prevalence of smoking is going down, and yes the prevalence of smoking related diseases is also going down.
SIDS has decined dramatically - but the big decline was caused by the Back to Sleep Campaign. We have succeeded in preventing SIDS, but this has increased the proportion of cot deaths that are caused by tobacco smoke (when pregnant and after birth).
In the case of heart attacks, people quitting smoking prevented nearly 30,000 deaths from heart disease between 1981 and 2000 in the UK.
Lung cancer has a long incubation period, so we recently reached the peak of lung cancer incidence - the numbers of people dying of lung cancer have started to fall, reflecting the decline in smoking rates over the last few years.
So yes, we expect that smoking related illnesses in general wil decline as prevalence falls. However, other threats, like poor diet and lack of exercise are likely to keep preventable mortality high in the West. And in the developing world, the tobacco companies are relentlessly pushing thier products, and tobacco-related illness is yet to peak. Worldwide, tobaco use kills more people that HIV-AIDS and malaria combined - about 5 million people a year.