Take a position: Brands cannot be expected to last forever versus there is no reason for a brand to ever become obsolete.
Often, after a brand begins to slip in the marketplace or disappears altogether, commentators observe, “all brands have their day.” Their rationale is that all brands, in some sense, have a finite life and cannot be expected to be leaders forever. Other experts contend, however, that brands can live forever, and long-term success depends as much on the skill and insight of the marketers involved.
First, I think you may need to define your time scale. For example, in terms of geological or astronomical time, humanity as we know it is a relatively short-lived phenomenon, and brands an even shorter one. On the scale of the universe at large, the answer is that no brand can be expected to last even as long as a short-lived star, much less the universe as a whole.
On a human time scale, again it`s important to ask what your time scale is.
The best way to answer a question about the future is to study the past. One starting point to answering the question of how long brands can last is to ask what are the longest enduring brands in existence. Many brands vanish because technology changes -- the most successful horse-drawn buggies, hand looms, etc. no longer exist as brands. While major world religions have endured for thousands of years, for commercial enterprises, 200-300 years is an exception.
If your timescale is a human lifetime, and you are asking about whether you can start out at Company X selling brands Y and Z fresh out of university and still be working with those brands until you retire, it`s possible but not probable, and varies by country and industry. You might want to research statistics on the average longevity of companies in different countries and industries to see what the chances are for a given one.