For one, NVIDIA's volume is certainly not a tiny fraction of AMD's. In fact, if you think about it, NVIDIA just might use more silicon real estate than AMD. NVIDIA might be a fraction of Intel right now, but certainly not tiny, and if it's Intel MCPs capture a significant share, it might not even be a fraction in the not to distant future.
For two, you seem to be missing some fundamental economic logic that pertains to fabs. A) building the fab is an incredible cost and this cost must be applied (also know as depreciation) to future production. B) TSMC has a very low cost structure. One reason is that it operates in Taiwan and China. Cheap labor. Another is that it spreads its fab investments out over long periods of time. It doesnt just produce on the leading edge and retool, it sells the older capacity to many many other firms for years years later. Sure, Intel may produce chipsets at .13 or something, but TSMC is probably still cranking stuff out at .35.
But the very same logic applies to owning a fab yourself - you can use "older" capacity for other products in house. You have to deal with fixed costs and depreciation regardless of who operates the fab. All the factors associated with TSMC's costs will be included in the price they will charge AMD for production, including depreciation - as well as their profit on top of this.
The cost per-chip with in-house manufacturing is:
(Fixed cost of fab)/(# of chips over lifetime) + variable cots
Outside, it is going to be:
(Fixed cost of fab)/(# of chips over lifetime)* + variable cost + foundry profit
*The foundry still has to charge that in order to get ROI.
The time value of money is going to be accounted for in either cases - AMD will still pay for equipment depreciation, only it will be some else's equipment. You argument is that it's better to rent then to own. It is actually better to own then to rent - the problem is, you may not afford to.
When you also account for points 1 and 2 in my previous posts, I think it's pretty clear you should own your own fabs if you can afford to and know how to run them efficiently. Companies like Nvidia choose to forgo the financial advantages of owing fabs in long term due to enormous initial financial burden building one would impose. The reason that they can do so is because their competitors were subjected to the same cost structure. A febless AMD would be less competitive with Intel then it is now, due to production cost disadvantage this arrangement would impose
In terms of a car analogy, a fabless company leases a car (fab capacity at a given process), and after the lease expires leases another one. Company with fabs buys the car, uses it until something better comes along, buys something else while still using the original car until it has extracted the maximum utility from it, then sells it or breaks it down for parts. You will get a better ROI in the latter case, but it does require you to have enough money on hand to buy a new car every time a new model comes out. Intel does. AMD may soon not have.