Please, explain how I'm wrong then. If you really think situations like this hurt Nintendo, then I think you're being ridiculous. And the idea of a company not wanting people to keep using the same product forever, but rather keep on buying new products is not a particularly new or radical idea.
You are not only wrong your argumentations are pure fluff based on pure fantasy
First you go on saying that "Speed running is generally a niche community" but then when they play a game they actually hurt Nintendo's business... it's either niche or relevant, it can't be both.
Then the fact that it hurts business because they do not buy new software is a thing that you invented on the fly because you essentially choose to ignore the fact speedrunners very often try to speedrun the sequels of games that they played for hundreds of hours before, i won't give you simple examples because almost every speedrunner does this, just look around.
Even if you don't count the above you still have a couple of prime examples that speedrunners not only help to sell new games but also to create them:
romscout, known Castlevania SOTN runner, was on the fundraiser stream with Koji Igarashi to help raise the money for Bloodstained which is a spiritual successor of the Konami series and guess how did he help? By playing alongside Iga SOTN, a 18 years old game that the runner played for his own admission thousand of hours (plus hundreds of hours to all the
sequels that have been released).
Another example is the Uncharted runner ovendonkey that has been hired by Naughty Dogs to be a playtester to fix bugs and glitches.
Oh and let's not forget how speedrunners like CosmoWright, Trihex and others were invited to sponsor the Nintendo World Championship at this year's E3 which was the best (only good) part of Nintendo's presentation.
So i just demonstrated how speedrunners: give money to software houses by buying sequels of the game they run, help videogame creators fund their projects by giving them exposure with their runs and help developers make better games by testing them.
Which is exactly the opposite of what you said.
Romhackers furthermore helped so much the franchises that Nintendo felt the need to use the same ideas of romhacks and actually make an official game around creating your own levels, if this isn't essentially helping them make more money i don't know what it is.
Now on the other side, how removing Youtube videos of TAS and romhacks damages Nintendo's brand? The very answer of this question is this thread, see how many people "in the know" are not happy to say the least about this course of action and think about all the fans of the contents that they are trying to delete from the internet, these kind of people are the greatest fan and technically Nintendo should keep them the happiest.
Nintendo might not care and is willing to take the hit but it's obvious that stuff like this put them in a very bad light, it may come the day where they might exaggerate and they are met with huge backlash from communities and have a global exposure about this stuff.