Leonardo would have done what everyone like him did around that time: convinced a wealthy patron to fund his experiments. If he had gotten many very rich nobles convinced as well, he may have gotten tons of money to hire lots of people to build and test various designs. We might have had heavier-than-air flight 400 years earlier. Of course, without powered engines, they wouldn't have had nearly the success they have today, but one can imagine some pretty elaborate gliders, or possibly hot air balloons, showing up hundreds of years earlier than they actually did.
So I absolutely reject your notion of "it would never have made any difference." The fallacy that history must always have gone exactly as it did is a common one, but it is a fallacy nonetheless.
> Likewise, let's say you're right, that conquering death is an inevitably solvable engineering problem. So what would you do with that knowledge, right now?
I already told you: organize a nation-wide (or international) Moon-landing level effort to make ECMO machines 100x cheaper, more portable, and more widely available by the end of the decade. Spend 4% of GDP on it. Keep spending 4% of GDP on similar research.
> Only if you knew the date of pending immortality was let's say, <10 years away, would you make some kind of drastic push to try and keep everyone alive barely long enough to benefit from that breakthrough.
It's not one date. It's a long path with many problems to solve along the way. It may indeed take hundreds of years to achieve complete agelessness. But every year that we wait to start (and we are waiting, because of our belief that it's not possible), is another year that people are needlessly dying. Every problem solved along the way is a few more years for many more people.
> I vaguely remember reading a short story about this very conceit,
Yes, bad sci-fi has used the inevitability of death as a plot element for a long time, along with the silly argument "do we really want to live forever!? BUT EVIL!!" It's never really held up to scrutiny.