No food could be preserved forever in their natural form. All natural foods are "alive" and, like all of the other living materials, are susceptible to processes of deterioration and decay. These gradual changes in fresh foods are due partly to chemical alterations in the living protoplasm of the food itself, generally catalyzed by the cell enzymes, and partly to changes brought on by minute organisms that get into the foods from external.
Meat, fish as well as eggs go putrid, fats go rancid, milk will go sour, fruits turn moldy or ferment, vegetables wilt and rot, cereals end up musty or germinate, mainly because of the action of enzymes. All natural foods are gradually and constantly changing in character as well as composition. Their visual appeal, smell, flavor and food value tend to be gradually being altered. The preservation of food consists in the halting or slowing down of these changes.
These changes usually are not always harmful or undesirable. We can see that cream is "ripened" prior to it being churned into butter, that cheese is permitted to "ripen" to produce flavor, and that meat is "hung" so it will be more digestible. However there comes a point in the decay of most foods when they become obnoxious or unsafe perhaps even, in some instances, poisonous.
It is certainly advisable to consume fresh foods whenever we can but this is simply not often feasible. Such a huge proportion of the world's inhabitants resides in big towns and cities far from farms and gardens that foodstuff needs to be transported great distances and kept in good condition for long periods. Some countries around the world produce more food compared to what they can consume themselves while just about every country produces surpluses of such foods as fruits, vegetables and eggs at certain seasons of the year. Modern civilization couldn't exist without using preserved foods. But for the use of methods of preservation much of the world's food would be wasted and our diets would be much less varied than they are. Preserved foods usually require less preparation and less cooking than raw fresh food and so have the merit of convenience. Their flavors are frequently altered but there is little loss of food value except for some loss of vitamins, particularly ascorbic acid and the B vitamins. Provided this is realized and fresh foods eaten to make up for this loss there is no harm in the increased use of preserved foods.