Spotify doesn’t need to “get better” - you’re paying the fee for streaming rights to an ever-growing content library. Same with Netflix, etc.

There’s also an unspoken but important accounting detail here - if you run an online software business and offer a one-time, lifetime purchase for your product, you are now contractually on the hook to provide that service to customers forever, even if your business stops growing, which means that the liabilities of your business are not capped. This is bad if you operate with a lot of cloud expenses, and it means that these businesses are unsustainable unless new customers keep coming in, whereas a subscription model scales with utilization.