If Build UI was still making content, they would keep getting sales. There are also other ways to implement a "pay once" model that is sustainable, but it involves designing a much more thought out product roadmap and gatekeeping features behind new major versions where you need to pay for an upgraded license.

Jetbrains has done this for decades now with great success and is the standard sales model for most freemium WordPress plugins. Heck, even Adobe had a similar model until they were convinced they could squeeze out even more profit by charging monthly and trapping customers into subscriptions with high cancellation fees (my words, not theirs).