I was kind of thinking this too. I've spent the last 5 years absolutely addicted to an mmo so having monthly patches or so was normal and something we generally looked forward to. You have to trust that the developers are making these changes in order to help improve the game. They typically aren't going to try to change the game in a negative manner.
I think also what people need to realize is the amount of involvement they have for a game you essentially are paying a one time fee for.
The base game of a mmo on top of expansions every few years on top of monthly fees to play...and you're paying those fees for the amount of work and upkeep they put into the game.
Even with NL there was content added later and DLC content released. So you got much more than your one time fee and the same really is for NH. Online is optional but even with the membership it's miles cheaper than any others. They've already shown to be releasing regular updates, have shown to put out fixes for bugs, and etc...clearly this game is going to have support by nintendo for a long while to come for the one time fee you paid for 60 (online optional but still 20 annual is nothing and goes to more than just this game).
Rare drops/spawns are supposed to be rare. There's many methods you can use in this game to make money, if you banked on that one method then that's your choice but it's also understandable why they would patch it.
So the tl:dr from all that is...if they wanna patch a spawn rate of rare bugs/fish then sure have at it.