Software is never done if the software it runs on is never done. Making that transition is not easy, and tech debt can accumulate to the point where you have to make substantial changes. In the meantime, UI standards/tastes change. Now you're at the right spot for a re-write
Its usually driven actually mostly by hardware. Which is driven by games (and now apparently AI).
Hardware gets better so UI gets more glittery, everyone tries to "stand out" by looking/working best, so the UI "taste" changes. Or hardware form factor changes, so everything needs re-written to support it, etc.
Next hardware re-write will be some mix of AI/low power. None of the "green" energy stuff will deliver in time, nuclear won't happen in a way that is both safe and any-time-soon, and meanwhile power-hungry AIs will be battling for cpu cycles and watts with other devices.
Your devices will be very low power and the majority of modern software will be laughably ill-equipped to handle that. Or it will run on giant mainframes that look nothing like x86 desktops or even server farms, more like specialized super computers.
