Aren't some trees, some fungi, some sharks and some crabs basically exactly that? They are most certainly complex life. Sure you are right heir metabolic profile is very different.
But really the your argument is already shifting to "there is no life ver similar to humans that do it, so it must be impossible" which imo is a much larger stretch then assuming it's possible.
We aren’t much more complex than a crab, if we are at all. “Complexity” is not what makes us what we are. It’s that we went down an evolutionary path that heavily leveraged intelligence and social cooperation so we got a big hypertrophied brain. Our brain is like a cheetah’s musculoskeletal system or a rabbit’s reproductive system.
The OP is also massively underestimating plant complexity. We aren’t much more complex than a tree either.
We are higher metabolism than both though, and with that the OP has a point. We are already long lived for a high metabolism animal. Our metabolic rate makes it harder for our repair mechanisms to stay ahead of oxidative and radiation damage. That will make extreme life extension hard for us, harder than if we were reptilians or arthropods.
But really the your argument is already shifting to "there is no life ver similar to humans that do it, so it must be impossible" which imo is a much larger stretch then assuming it's possible.