When the old well dries out, the resourceful one will dig a new one.
Imagine you have been doing something really lucrative for a few years. It may be some kind of business or it could be a certain specialized trade that not many were practising. You made lots of money and naturally when a certain business idea starts to make it big, others will join in the bandwagon to take advantage of the trend.
A classic example is the cell phone business. When cell phones were introduced, those with the foresight saw a big thing emerging and cell phone stores were started. The pioneers made their money within a few short years and as it is now, cell phone stores are everywhere. Those with good financial backings quickly overtook those who were stagnant and not innovative enough.
This are the kind of business that prosper with trends. The fact that you see new cell phone models coming out every two or three months is because manufcturers are trying to outdo each other as trendsetters.
It is the same as doing other business. If the competition becomes too stiff, we should be looking for new innovations to get back our market share. It does not necassary mean abandoning the old business but improving on it so we stay ahead or our competitors.
However if you really feel that here will be not future anymore in your line of business, it will be advisable to sell out while there is still some value. If you keep hanging on, one day, you will find that you have a dying business in hand that you will never be able to cash out.