What would be the point of nVIDIA coming up with anything new if the core technologies they develop can run just as well on their competitions cheaper hardware? Why would anyone buy nVIDIA cards after that? What would drive nVIDIA to innovate? How long do you think nVIDIA would last doing that? Oh right, you don't care, because you are so short-sighted.
First, you state that both companies should build walled gardens, each screwing over the other's customers, instead of keeping the PC platform open. Then, you call me short-sighted. This is absolutely hilarious.
Monopoly is thrown around too freely it's lost all meaning.
That's not freely throwing around a word. If AMD goes bankrupt or leaves the dGPU market, that's going to be an actual, literal monopoly for Nvidia. There will be nobody else, and I assume you have the least ammount of intelligence to understand why that's a very bad thing for customers, don't you?
Also, you don't even need to go as far a literal monopoly to see why the effects of no competition are bad. Could you imagine if, for example, a company charged a $450 premium for a CPU for a 100 MHz increase in frequency on the exact same silicon? Look at the i7-4930K vs. i7-4960X. The only reason Intel can do ridiculous stuff like this is because they don't have anyone to keep them in check at that performance range, so if you need that kind of power you have no choice. Do you think that's a good thing for the customers?
Is Apple a monopoly? Because they charge more than anyone else for their stuff. Windows is a supposed monopoly, but I don't remember them charging unreasonable prices for their software.
Monopolies have nothing to do with charging more than the competition. Apple has plenty of competition, in fact. If you need a phone, you also have Samsung, HTC, LG, Motorola, Microsoft, Blackberry, Xiaomi and several others. If you need computers, you have Microsoft, Dell, HP, Lenovo, Acer, MSI and several others, including the option to build your own. Microsoft is not a monopoly either, because they not only have competition from other OSes (OS X, Chrome OS, other Linux distros, BSD), but some of them are actually free. On the dedicated GPU market, there's nobody else besides Nvidia and AMD. Take either of them out, and it's a monopoly.