When you're a startup, you do something new and (hopefully) do it well. You get customers and you grow.
Eventually you become an established company. You do your thing really well, and customers are happy to pay you the price you ask. You make a healthy profit margin and times are good. Competition takes notice.
You keep doing things well, but your competition has a second-mover advantage. They get to work with newer tooling, no legacy codebase, all the benefits of a fresh start. They start to win your customers away from you.
You're doing things well, but it no longer really feels well, because your success in the marketplace no longer correlates with the maturity and quality of the product you're offering. You may have an incredibly mature and well-supported product, but it feels stale. It turns out that customers appreciate qualities like "refreshing" and "modern" that your product seemingly lacks.
As you start to lose customers, your declining revenue forces you to lay off employees and your stock starts to decline. Without a big change, you go out of business and shutter your doors for good. The executives are forced to sell the IP for pennies on the dollar to some patent troll as part of the company's bankruptcy and liquidation proceedings, and might open-source the IP if no buyer can be found.