Could you intentionally harvest all of the donor area hair via FUE?

Someone with a hair loss pattern may be perfectly happy with 1000 grafts, while others may not be happy even with over 3000 grafts.  You also have to realize one surgery may not be enough. To add to the confusion, straight black hair on fair skin results in a thinner appearing outcome then someone with curly blond hair on fair skin.  I hope you can appreciate there are many factors to the end result than just a "number" of grafts.
If you harvest all of your donor area hair, there will be a lot of scarring and is not a result many patients are seeking.

There is no way any good Doctor would risk trying to harvest every hair from the back and sides to gain coverage on the top.  The risk in over harvesting is creating a completely scarred up donor area that is in theory creating a problem to fix another.  There are ways with a good plan for someone with that much hair loss to get good coverage without jeopardizing the integrity of the donor area.

That is a terrible idea. Creating a bald side and back of your head that is full of scars, is not my idea of something any reasonably ethical surgeon do regardless of how much money you would pay him/her