In many ways you're right BUT your dog was only born 8 weeks ago and retrieving a ball isn't that important a skill in the scheme of things.
What is important to teach at this age is sit, wait, come and walk to heel. And get those commands absolutely bullet proof which will take quite some months.
I was thinking of the other thread about your dog too, in the back of my mind and had an overall feeling that you may be expecting too much, too soon in terms of adult style play.
Just for info - I have recently seen a series of X-rays taken of a dog's hips, starting at 8 weeks and finishing at about a year old. At 8 weeks there are great areas of bone which just haven't yet formed around the hip sockets and there is significant scope for damaging these very important joints. Things change massively by 20 weeks, but despite a very interesting thread on here to the contrary some months ago, I remain convinced that great care needs to be taken with a young pup - particularly a big breed dog.
So, no coming downstairs, no jumping out of a car boot, avoid as much big strain and impact on those joints as possible for the next few weeks. It doesn't mean get neurotic, or stop her being a puppy but carry her downstairs, and lift her out of the boot for example.
Maybe roll the ball about for her to follow this week, and up the game a little more next week instead?