If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed), then why is it safe flying in tailwind, such as in jet stream?
I once had a flight instructor tell me, "As soon as you get airborne, there's no wind." While this is not strictly true in all cases (wind shear is still a thing), it does hold true for flying with the wind. The aircraft is moving in the moving air mass, so the fact that the wind is "coming from" behind you only helps you get to your destination faster. Aircraft can fly at the same airspeed no matter which direction the air mass is moving.
“Wind” is simply an air mass moving over the ground. When flying, planes do not experience wind per se because they are flying at a (mostly) fixed speed relative to that air mass.
We speak of a “headwind” or “tailwind”because the movement of the air either reduces or increases the plane’s speed relative to the ground.
We want to takeoff and land with a headwind because the reduced ground speed means we need less runway to achieve the same speed relative to the air, whereas a tailwind means we need more runway.
If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed)
This is incorrect. The airspeed required to take off is the same, no matter the speed of the wind relative to the ground.
Taking off in a tailwind is potentially unsafe because the required ground speed is higher. If you take off in a tailwind, you may run out of runway before your airspeed is high enough.
You cannot run out of runway while in flight.