But if the organism lived longer it would keep pumping semiclones. And if organisms are basically huge self-sustaining feedback loops, why won't they keep sustaining ... forever until some pressure forces them to decay
Because the semi clones are a full process reboot. Rebooting is an easier way to solve the problem of continuing to exist into the future.
You can search the space of evolutionarily adjacent entities faster by semicloning once and letting the fittest of those semiclone further, compounding the mutations. Such a strategy will likely out-compete a slower searching one into extinction.
Sexual reproduction, which increases fitness? So theoretically the offspring are more fit clone producers so competing with them is bad, M’kay?
But who invented fitness? Theoretically a self-sustaining chemical reaction can go on forever, and it is energetically less costly to search ways to sustain it even further rather than rebooting the whole organism from start. The invention of death must have happened before the beginning of evolution. Which reminds me that our quest for immortality is the end of it.
There are trade offs for living longer. For example, children of older fathers are more likely to have birth defects.