This assumes a waterfall approach to development which implies multiple 6 month to year long development cycles.
In reality, a mature stable project can receive monthly updates, and an immature half-working project can be in maintenance mode. Furthermore this may work for software that should be seen and not heard doing its job in the background without much user interaction, but for software that users interact with regularly, the design needs to be periodically refreshed to match current trends or users will leave for the newer sexier product with fewer features. We've seen this time and again. I have absolutely experienced a mature product that was "finished" (abandoned) like 4 OS version ago that just doesn't run/work on the current OS version because the platform has added new security controls, APIs, and/or UX expectations, etc. No amount of security updates would fix that.
So while I understand where you're coming from opining for a world where we ship mature software and security updates only, I don't think it's remotely realistic given the way humans operate.