A gazillion hashes/second doesn't make the network secure and in fact the number of hashes is completely irrelevant. What matters for the most part is the amount of electricity used and secondarily the cost of the mining equipment. For example 100 watts buys you something on the order of 200 GH/sec of SHA256D. The same 100 watts would buy you perhaps 300 hashes/sec of (to pick one with which I happen to be familiar) CryptoNight. These are approximately equal in value for mining; 200 GH/sec of one algorithm is not 2/3 of a billion times "more secure" than 300 H/s of another. These units are incomparable.
The current bitcoin network is about 150 megawatts. That's something on the order of a 1-2 million PCs using a CPU algorithm. At least until specialized hardware and mining farms developed, a bitcoin forked to a CPU/GPU algorithm would look a lot like a million bitcoin users all mining on their computers. Sounds pretty darn secure to me.
(Is this your "technical objection"?)
You are
assuming that 2 million bitcoin users would all rather upgrade to the "rebel" chain and start mining on their own CPUs rather than accept the cartel's proposed change to the protocol.
No I'm assuming that given the choice between software upgrade A, from some foreign source, that modifies the protocol in favor of some other interest group, or software upgrade B, from the same place they got their software in the first place, that protects their own interests, they would choose B.
As for who would mine on their PC, that is simply incentive based. The same 25 BTC are being produced every 10 minutes. If running a little program on your PC allows you to "generate free money from the interwebs" then, sure, many people would do it. Not only many of the existing million or so BTC users but many other as well (historically BTC mining booms have attracted new users -- as expected via greed -- especially during the CPU and GPU eras).
Given 25 BTC being mined every 10 minutes, profit motive will attract people to mine on their computers, mine on the old computer in their attic, mine on their friends' computers, buy new computers, etc. until approximately the same 150 MW is mining as before. As for cloud mining, I suggest you do some industry research to find out how much cloud computing capacity is available and how much it costs. You will find that your assumptions about what is available to be grossly inaccurate, if you think cloud mining is a serious threat (in fact, it would initially be more of a source of honest profit-motivated mining, before becoming uncompetitive).
Please redo your calculation after recognizing that even if only 10% of users get involved, at all, with mining, many of those will get involved on a significantly larger scale. At the low end that means 2-10 computers, or building GPU rigs with a half dozen GPUs (each roughly equivalent to a desktop in capacity), etc. At the high end it means semi-pro to pro-farms with dozens of rigs, initially, even larger over time. Oh, and let's not forget botnets. They'd also join the party, bringing many more computers to grab their share of the "Free Internet Money!"
This is not theoretical; every aspect of what I wrote in the previous two paragraphs
has happened during every BTC mining boom and every altcoin mining boom. Your claim that it wouldn't is backed by no historical or other evidence whatsoever.
Let me be clear about this -- I don't expect any of this to happen. I won't happen because everyone -- including the miners -- knows that it could and would happen if a mining cartel tried to exercise the power you claim they have (but actually don't) to fork the protocol in its favor. Therefore it won't happen.
If a mining cartel does indeed fork the protocol in its favor as your model predicts, then I will come back here and eat crow. If it doesn't happen (over some reasonable period of time -- say by the next block having or the one thereafter), will you do the same? As you know I would prefer a substantial bet, but you apparently are either not confident in or insincere about your statements or don't like the idea of receiving free money from a stranger on the internet.