>because it increases overall blood supply

But that's what I want explained why is more blood beneficial for a person's forearm?

I don't feel like I am lacking blood flow during my daily tasks in life. Can "extra arm artery mutant guy" out-compete me in a specific task so critical it means his genes are more likely survive than mine are?

I would guess extra blood flow could somewhat prevent people from suffering of cold fingers. I couldn’t find evidence for that, though.

Worse, Wikipedia claims it’s easier to commit suicide by slashing your wrists if you have this artery (https://en.wikipedia.org/wiki/Median_artery), and https://radiopaedia.org/articles/persistent-median-artery-of... says it can be a cause of carpal tunnel syndrome.

I don’t see how either could be an evolutionary advantage.

More stamina when composing romantic emails → greater chance of reproduction.

Replacement makes sense, your friend with extra artery will survive in case of a critical surgery and might have another kid while you'd die with no extra kid. Does not need to happen to you but on average. Your friend with better bloodflow might also have a stronger erection and therefore on average more kids (pulling this out of my ass obviously). You dont need to have any obvious disadvantages, on average it might still work better..

If we're talking about a trend strong enough that most humans will have this trait in 80 years, then if natural selection is driving it, none of those advantages are great enough to explain that.

The most likely answer is that natural selection isn't directly driving this specific trait.

That’s not exactly a selection pressure. It would seem likely that there is no selection pressure here but in fact greater genetic diversity due to overall better survival of more and more people.

Perhaps those that developed differently to have this artery retained were selected against in less civilised times.

But that does not increase, in any way significantly, a person's reproductive success.

That second part seems unlikely to me. I would guess the percentage of people that’s beneficial to is extremely low.

So this means evolution actually reacted on progress on medicine and we are evolving our own spare parts for surgical procedures. While this makes sense im still amazed!