You have made some very reasonable comments. Some counterpoints:
On the "repricing":
The post I was commenting on seemed to indicate the they were unhappy because NCL's DSC is higher than most cruise lines and, apparently, did not offer such exceptional service as to warrant it (again, compared to the competition) (also, reading between the lines here). So, by reducing the DSC because it was considered unreasonable compared to other cruise lines, rather than due to outright poor service, he decided to cut the fee and reduce his trip cost to the amount he felt was proper.
It's different than in a land based restaurant because there, you pay X% of a meal price.
What percentage are you paying the dining staff? Would you care to guess at a meal price for each meal? Worse than that, what percentage do you give to a room steward? Impossible to do it that way. You might come up with a per diem you think is fair - but that only works if you cancel DSC completely and tip individually in cash. Fine for the steward...but not so much for dining staff as well as the others behind the scenes who share.
So, there are many differences here vs. not-at-sea gratuities.
And, unless you have universally poor service onboard, reducing the fee due to one or two poor performances barely touches those by the time allocations filter through. Better to praise or criticize individuals in writing.
I don't sail as often as many of the CC posters, but have cruised on and off for over 40 years.
The cruise lines converted their old tipping system (first suggested amounts; then pre-billed but adjustable) to the DSC of today. In that regard, I believe they left open the adjustability to avoid making it outright mandatory.
If it became mandatory, it would serve no purpose to exist as a separate charge any longer and could be rolled into the fare and forgotten - other than they currently can promote the cruise at a lower price (before DSC is added, let alone "taxes & fees").