That is the point: users who do nothing automatically accept any soft-fork type of change, even if they are unaware of it.
In your example users who chose not to switch over to the invalid alt software would definitely not accept those coins. You just admitted so yourself.
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...
Or the merchant could simply suggest they don't accept the new alt, patch their wallet, download the High inflation wallet on another device (for security) to conveniently refund the few customers during small window when the mining cartel instituted the changes. The merchants coins are not tainted at all as all they would be doing is refunding an alt on another forked chain. There would essentially be 2 forks created.
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.
Incorrect. Has the longest chain of
Valid Proof of Work. Lifting the 21million limit would instantly make that chain invalid in the hearts and minds of most bitcoin users. There has always been an authority who decides what miners are good and bad , the users. Bitcoin has always maintained as being an open source project where even individuals can veto miners and other developers and fork off on there own. Where do you think most the alts came from?
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...
Every user can individually make that decision for themselves. Their vote can be made with as little of effort as inaction. Ultimately users control their currency. Of course their currency may or may not be worth more dependent upon how much of the community follows them but the more important principle is the user is in control of their currency and choosing to branch off on another chain doesn't even necessarily have to hurt them financially as they could recover their investment by slowly or quickly dumping their old coins while simultaneously using the new coin.