This seems fallacious to me, the scenario you describe has zero opportunity cost for being alive till 200, it's free. Assuming no afterlife, you gain nothing by artificially limiting your lifespan to less than half its amount.
But in reality, there's opportunity cost to trying to extend your lifespan, it's all the money and attention and other resources that could have gone to improving other much more certain-rewards problems.
In other words, your thought experiment is a fully general purpose argument for any investment. It's conclusion is that you should _always_ go for the investment no matter what the opportunity costs.
Example (1)
- We should expand the data center from 80 servers to 200 servers
= Why? our loads are not high, we should instead focus on improving the product
- Well imagine if we already had 200 servers in the data center, would you scale it down to 80
Example (2)
- We should invest in renewable energy
= Why? nuclear power is much more certain and mature
- Well imagine if we already had renewables, would you phase it out in favor of nuclear?
In both cases, you're completely missing the point of your opponent by the "Imagine if we already had" maneuver, we don't, that's why your opponent is against it, it's a risky investment with uncertain rewards and plenty of same-utility alternatives (in the opponent's view).