As a product gets better, the additional, functional benefit of each product improvement tends to get smaller. Certain product functions become good enough. Continued improvement of these good enough functions: (1) isn't valued as highly by consumers, and may not be enough to convince consumers to upgrade to the latest product version; and (2) leads to over-complex, overserving, overpriced products that only appeal to the most demanding users, ultimately hurting a company's sales and profits.
It also seems like products hit a point at which they can't be meaningfully improved in their current form. When this happens a product's form/shape has to change to stimulate sales. PC's are good enough and hard to improve further, leading to stagnating PC sales, yet computing products have continued to grow through different forms like tablets and smartphones.
Product functions that aren't good enough are the ones a company needs to focus on, because improvement in these areas motivates consumers to buy the improved product.
When functionality is generally good enough, it's time for a company to: (1) start improving product access and affordability, making it easier for consumers to acquire and use the product; and (2) start exploring new product forms where improvement will be valued.