I could come up with two rebuttals

1. The event where we live forever but are unable to 'crack' happiness and are stuck in an eternal, miserable chase for it. That seems like an unlikely scenario to me given a span of thousands or millions of years. And at any point in time where we 'crack' the code for great happiness, the reward would still be far greater than anything we might have had to go through.

2. For the long run, both life and death offer the same complete uncertainty. If you continue to live indefinitely you'll experience things that are beyond the scope of anything you can understand or perceive (the end of the universe etc...) so even if you're in a state of bliss for a while at some point you might experience things that are terrible beyond anything imaginable.

I was a bit hasty when writing the original post, so please allow me to clarify what my exact dilemma is.

For the sake of the argument, please accept that at least within some highly achieved academic circles, an unexpirable human body is thought to be at least within the realm of possibilities. Many smart people out there think we could make it happen soon.

Please also accept, that given a very, very long period of time, we could acquire the power to engineer our own feelings and emotions and to open up a wide variety of subjective experiences. Just imagine antidepressants or neurological interventions but with a much greater potential.

Given that the ultimate goal is to be as happy as possible for as long as possible, wouldn't the rational course of action be to maximize the chances of experiencing that utopian future while be willing to sacrifice happiness until then?

In the event of success, the reward is beyond any suffering that might have been incurred. In failure, that is death, non of that would have mattered as I no longer exist.