When does an archer put down his bow and grab a sword?

[Eth]

but it's surprisingly fast with all that proper gear, feats, & with high enough DS% really myth-shattering.

There are no myths. You could simply use math and see what's better.
You know why people don't run doubleshot archer builds? Because they don't produce as many attacks per second as a 10k star build. As simple as that.

On a 50+ Wisdom build with 10k stars you produce around about 1.2 extra arrows per shot during 10k stars.
10k stars has an uptime of 50%, which translates into what would roughly be 60% DS if you simply cycle it everytime it comes off cooldown.
In reality that's higher though, since you'll typically use it in situations where it matters and not hit the button every time you are off cooldown.

Now how much doubleshot can we have?
9 PL + 8 item + 5 shadow arrows + 20 capstone + 3 elf + 10 feat = 55 sustained
Now you can add killer for 0 - 20 extra DS situationally.

Why is DS still worse? Because of manyshot and manyshots DS penalty, which cuts down 58% of your doubleshot uptime.
So instead of 55% - 75% DS your actual average DS is only around 23% - 32%.
A 10k stars build doesn't really care about that, since there are only 15 seconds in a 120 second cycle where DS would even apply.
But whatever DS bonus you have would apply as some extra benefit in those 15 seconds.

Not using manyshot because of this doesn't help either. I don't have accurate numbers what the attacks per minute of a bow user with current alacrity bonuses is, but unless you have more than 80% DS, you should always be better off cycling manyshot.

Now if the DS penalty would be removed or cut to some degree, this would be a whole different talk, but as it stands currently, pure rangers are just far behind.

This is not any different on your own unique "rangerdanger" build.