Solving a problem starts with seeing it as a problem rather than an immutable fact. And "suppose we didn't have the problem" is a thought experiment. In this case, it helps show the moral equivalence between killing people and stopping people from living longer.
To be clear: I do think it's a bad idea (or at least fraught with all manner of peril), and I do want to stop people, but not by force, but rather by persuading them that it's a bad idea, which is what I'm trying to do here. But if I fail to persuade you, I'm not going to stand in your way.
And I would like to persuade you to stop that, because I believe that in doing so you're making it marginally more likely that more people will die.
What would it take to convince you that whatever problems you're envisioning, they are not worse than 150,000 people dying every day, or 100 people ever minute? Take whatever value you attach to an individual specific identified human's death, say someone you actually care about dying rather than just a statistic, and try to imagine that happening almost twice a second, forever. There are problems that are still worth considering in comparison to that, but they're few and far between, and they all also involve human lives (e.g. "making the planet uninhabitable"), so they need solving at the same time and nobody is arguing for not solving them.
> What would it take to convince you that whatever problems you're envisioning, they are not worse than 150,000 people dying every day, or 100 people ever minute?
You'd have to start by convincing me that death is unconditionally bad.
If you could dramatically improve the quality of life for a million people by prematurely killing one, would you do it?