About a re-org: the extra work put in by the re-orging miner matters, but also the economical disruption (or improvement) caused by the event.
Consider a miner who tries to do frequent deep reorganizations on the chain, let's say 6 blocks, for whatever reason.
He has to have something to gain by doing it, so let's assume he's somehow incentivized - either because of gaining the block rewards+fees, or double spending..
As long as other miners feel that the benefit to be gained by the re-org (e.g. increased proof-of-work, significant fraudulent activity eliminated etc) isn't harming ...
...but *helping* the network --- they might support the re-org instead of considering it an attack.
However, ...
... other miners are under no obligation to support any type of re-org.
Under normal operating circumstances, it inconveniences users - at worst it's an attack (see the whitepaper).
Merchants will be inconvenienced when their payments get reversed.
Absolutely - I group everyone that's not a miner under 'users' above, as in 'users of the blockchain'.
End of this little monologue - thanks for reading this far!
