Relevance of our products/services to customers

I received a comment on my previous blog on sales process that if there is no demand for a product/service, improving the sales process might not have any benefit. 


True. But how do you know if the decline in sales is due to lowering demand for your product/service or is it due to some other reason. 


My belief is that as long as your product/service is able to solve a key challenge for your customer, the demand for your product/service should not decline. If it still declines, the culprit lies in the process. 


A follow-up question is, how, when and how often do we check if our product/service is still relevant for our customers. Do we have a process in place for this ?