Remember when a company goes out of business, yes the business ceases, but the people trying to make money off of it continue to exist. They can start a new company or just get normal jobs or they can retire. They have lots of options. Any business venture has to be more fun or more profitable or somehow more valuable than something else that they would be doing.
If you consider there to be unlimited value in paying ransomware, you would also see unlimited value in buying said business. But that's not how businesses are evaluated.
That's actually a good way to think of it.... if you had to buy the same business fresh, how much would an investor be willing to pay? You would never pay that same amount in ransomware because of the additional risks involved (they might not turn over your data, they might have sold it, you are now a target, etc.)
Because that is basically what is happening with ransomware - you are buying the business again.