> That's completely missing the point of the exercise
No it's not, you're just too invested in your example to realize there is no point to the exercise you proposed. A model where up to 90% of data (the "unexperienced" part of your life depending on age) is imagined and by definition inaccurate can't be used for any determination.
> The point of the exercise is to figure out what you want in life now
That's the point of the exercise you proposed, very different from a deathbed perspective, and one I believe couldn't work as described.
The deathbed perspective is an exercise of taking your entire life experiences and distilling them to get a concrete core as seen from the "final checkpoint". The key is knowing that there will be no future experiences to alter the result, and that makes the determination as final and as accurate as it can ever reasonably be. It's not a decision making tool, it can't be used to alter the future. It's a conclusion, a punchline.
But if you want to see what's important now that's a decision, a seed for your future, and it's open to change and evolution. This cannot be more different from a real deathbed perspective. So taking this decision by imagining one of infinite possibilities of your future up to your distant deathbed makes no sense. Your subjectivity will lead you into imagining a completely different future every time, and getting a different perspective and decision.
That girl you like? Deciding she's "what you want now" is just a matter of imagining a future deathbed where she was key to your life happiness. Or maybe it's that job... Or something else entirely. When you're 20 or 30 your "old age" deathbed could have you a billionaire, homeless, or anything you can imagine. An imagined, relatively random, and distant future outcome can't help you decide what's best in the present more than flipping a coin will.
At best you can imagine you are now on your deathbed and go through the exercise to take a decision as if it were the conclusion. I still think it's pointless because imagining you're on your deathbed is like imagining a new color.