With a soft-fork, users do not have this choice. Even if 45% of the miners hate the change to the
rules, they cannot force a split of the chain, and must adopt it. The users will have to accept it too,
whether they are aware of it or not.
Users never have to accept it and can instantly veto any SF or HF introduced by the miners simply through inaction.
That is the point: users who do nothing automatically
accept any soft-fork type of change, even if they are unaware of it.
When that happens, the receiver would have to download the new wallet software to get access to those coins
Downloading a wallet to refund some counterfeit altcoin doesn't necessarily have anything to do with the original bitcoin chain or software. There would only be a problem if the user or merchant wasn't bright enough to actually check or ask why the coins weren't accepted before swapping out their old wallet.
That would be the position of a fanatic receiver: "As a matter of principle, I do not accept as valid those extra reward coins, nor any coins that were tainted by being mixed with them, no matter when or by whom." But of course the newbie who sent him those coins will not have to agree. If he did not receive the corresponding goods yet, he will want his coins back, "or I will call the cops". If he received the goods already, he may say "I have no other coins, I got them from my exchange and are good for other merchants; if you don't like them, it is your problem".
It would be like if a merchant rejected any dollar bills printed after 2008, "because they are fake"; and also any pre-2008 dollar bills that the customer may have got as return change when he bought a coffee and paid with a post-2008 bill. If there are many who take that stance, it may make some sense. If only a few do that, it would be just stupid...
In unlikely even such a scenario existed [ ... ] developer would likely implement some emergency patch to disqualify the mining cartel trying to hijack bitcoin.
They could do that, but the users would have to download that software to get back the old rules. More crucially, it will violate the most basic priciple of bitcoin. Namely, there cannot be any authority that decides which miners are good or bad. Anyone can mine, and the blockchain that you should use is the one that your client software accepts as valid and
has the most proof of work.
this is no different than a 51% attack
Indeed, there is no technical difference between a soft-fork and a 51% attack. Both are changes to the rules that are deployed without users's explicit consent or cooperation, exploiting the "majority of work" rule.
The difference is only whether the change is considered "good" or "bad" by the community; but that may depend on people's relation to bitcoin, the context of the change, how it is presented, etc.
Take my favorite example: postpone the next halving by 2 years, but then shorten the halving time to be every 2 years instead of every 4. This change would not increase the 21 million limit; it would keep the current rate of inflation until 2018, but the inflation would drop much faster after 2020, so that issuance would be complete in half the original time.
Would this change be "good" or "bad"? All the miners should love the idea, since it postpones the 50% drop of revenue next July. Holders who were hoping to cash out in 2019 may hate it, but those with a longer outlook may love it. And the miners could point out that, without that change, many of them would have to shut down, which would cause the hashrate to drop, which would be bad for bitcoin's security and very bad for its image...