You are confusing the mechanism by which the fork is triggered, with the economic pressure put on miners afterwards. You are confusing the pre-fork with the post-fork. It costs miners nothing to brand their blocks "version 4," whether they are actually going to adopt the new rules or not. It is an entirely different story when they start minting blocks that other nodes cannot validate. In the post-fork scenario, there will be two different versions of bitcoin for sale at two different prices. Miners will choose the most profitable coin to mine, just like altcoin miners shift around to different pools depending on changing difficulties and prices.
what you don't seem to understand is that the switch from "0% of block advertising gigablocks" and "95% of block advertising gigablocks (*)" will not happen overnight.
It will take months, if not years. The discussion will continue, and each and every one of the (very few) miners that are actually mining (not "hashing") will make the conscious choice to accept the gigablocks or to not accept them.
The ones "hashing" (not mining) have more power in that relationship. If a "hasher" doesn't like the version his pool is branding, he can just move to a different pool. What you "don't seem to understand" is that "discussion" and "conscious choice" among poor people is not what causes these kinds of changes. The investors that brought you the 300 dollar bitcoin will not abide USGavincoin, and changetippers alone cannot make up the difference.