The majority of carpet replacements have been due to carpet destruction (burn damage, tear,) because the carpet had become too dirty (lack of maintenance), or because of a change of design due to remodeling.
Matting: Once the pile begins to lay down due to heavy foot traffic, there is nothing you can do to get it to stand up again.
Stains: Even your carpet has a stain-resistant coating, this layer gradually fades over a specific time and leaves your carpet vulnerable to stains.
Bad Smell: If your carpet preserves an unpleasant smell even after it has been cleaned, it would be better to replace the carpet.
Padding: It is used to provide a cushion and to make the carpet comfortable to walk and sit on. It works as a noise buffer and as an insulation for the floor.