If it doesn't take 100 years to complete, then it doesn't take 100 years to complete.
But it is a sort of good point that a 100 year runtime in 2020 might turn into a 10 year runtime in 2030, making it only a 20 year runtime overall. For some jobs, a 20 year runtime might be acceptable, but probably those jobs are rare.
In general if a program takes so long to complete that its output is no longer useful, that limits its practicality and utility.
