To be clear it's not unstopable but in spite of things like fishous rend, which Arcanine is just as vulnerable to, Arcanine already is the 2nd most used pokemon in the format at 35% usage. Incineroar is so much better in its role it may overtake Togekiss for the top spot. Dynamax may keep it from being on 80% of teams like it was in the past but it's already almost guaranteed the 30%+ which Arcanine already gets.
Also, not wholly better then Arcanine? Hard disagree on that. Anything you'd use to counter Incineroar would be used to counter Arcanine but Arcanine is actually slightly less bulky so it's worse off. Meanwhile Incineroar gets a great support move in parting shot which lets it cycle another intimidate later as well as Fake out.
Fishious Rend on Arcanine isn't necessarily the issue.
Arcanine can get a Will-O-Wisp off to help its partner and reduce the power of Fishious Rend/Dracovish in general. Incineroar can't do that.
Both Incineroar and Arcanine are getting KO'd by Fishioys Rend, but Arcanine can get a Will-O-Wisp off and lower the threat level for the rest of the team. Incineroar can't do anything to Dracovish or Dracozolt.
The proliferation of Competitive and Defiant in the current meta also serves to weaken it, just as it has Arcanine, but even moreso, because it will rely on stat drops more.
I don't disagree that it will largely replace Arcanine on most accounts, but Arcanine has uses that it doesn't (primarily Will-O-Wisp support), and the meta, as it is now, deals with it better than in 7th gen.