By your statement you are clearly speaking on something you have not experienced. This is not flex. This is complete free play. It is not the magnesium, it is the hinge. If you have experienced this issue, you will know that it is not as it was intended. I was told just that by Microsoft themselves if you still don't accept this.
A question. Does yours sound like a musical rattle if you shake the device while the kickstand is open? The one I tried at the store certainly didn't do this in any shape or form. It was exactly as the kickstand was designed, it held its position. Not rattled about like a fisher price toy. I am watching top gear on my TV right now, if I shake my sp3, I can no longer discern the words being spoken. That is how loose it is. Its not simple flex as would indeed be expected, it is the hinge, they are loose.
I bought my SP3 i7 256GB at Best Buy. They had to order it for me, as they don't stock the i7's at all. When I got it home, I saw that it also had zero resistance between closed and the point where the resistance is supposed to increase significantly (30-degrees or so?). If I let go of it before that point, it would just fall shut.
Not wanting to wait another week for them to order a replacement (and considering this unit was already an upgrade/replacement for the i5 I had returned), I took it to a Microsoft store. At first they said that, since I was within the first 30-days of ownership, I should take it back to Best Buy. I told them that Best Buy didn't stock them and would have to order a replacement. They basically said, "Sorry 'bout your luck."
Then I asked them, if I have a problem within the first year warranty period, don't they take care of that on-site? He said yes, and, seeing where I was going with that, went to talk to his manager again. His manager told me he would take a look at it, and upon seeing the behavior of the kickstand, agreed to swap it out for me on the spot. He did not, however, seem all that happy about it. I simply told him that it's not within spec (as evidenced by the several we checked on display) and for $1500 I just wanted to make sure everything was as it should be. They gave me a new, sealed retail box, and I gave them just my tablet. I ended up with a free spare power supply and pen out of the deal.
The new unit does have a little play, but the kickstand basically stays at any angle I set it. I'm a happy camper now.
My current 512GB has the play/looseness in the first bit of travel that some of the previous units I had didn't, but honestly it doesn't really bother me. At any of the angles where the kickstand would fall shut, I don't really find usable angles of the screen. I have MS complete so presumably I could get it addressed sometime into the next couple of years. After going through 3 SP3's trying to find one that was perfect, this one is holding its own so far and I'm going to just use it and enjoy it until enough issues crop up to warrant having MS take a look at replacement under Complete.