I don’t think dead is really a fair term. For one of implies a finality that clearly wasn’t the case. Even aside from that, when Spock was dead would it have been inappropriate to try and recover him? What if doing so would cost two more lives? What if he and someone else had been dead and recovering them would cost one life? The needs of the many and all that.
Your question is both fair and unfair. It’s like asking when someone’s death is no longer tragic. News of a child dying is generally referred to as a tragic event. Is it still when that person is 20?
Another factor to consider is how Tuvix’s life would progress after declining. There’s at least one obvious and one slightly less obvious bits that come immediately to mind. Imagine your among the crew and someone important to you was lost in such a transport accident. Can you honestly say you’d treat Tuvix the same after you found out he could have brought them back and declined to? Just from an ordinary person stand point that’s already hard, but lets add in the other part.
How many people aboard Voyager, or any other Starfleet vessel, wouldn’t lay down their life for two of their crew members, even if they didn’t really know them directly? It’s an even bigger issue for Voyager because they’re stranded. After everything happens if it comes to a desperate situation, would you be sure Tuvix would do the same for you, or do you think you might worry if he’d be looking out just for himself. I’m not claiming such a thing is right or wrong, but it is human (and many other races) behavior. It’s entirely possible Tuvix would have been spared only to be a pariah. At what point is it not worth it?