I likewise don't think destroying the ARM ecosystem is Nvidia's primary objective here - far from it. It might not even be a secondary objective. But I do think the ARM ecosystem will be slowly torn apart either as an innocent bystander or as a less prominent secondary objective (given real-world complexities, probably some combination of both). When Nvidia buys ARM, and they explicitly buy them to improve their competitive position, there's only one direction where I see the incentives going. Those will be against the open ARM ecosystem, which is a breeding ground for big and small competition against Nvidia.
Even if Nvidia's primary motives are relatively benign, I think they'll inevitably create a sitation where the ARM ecosystem can't continue existing in its current form. That's where the real tragedy will be.
Buying ARM not only allows them to control the direction of development, it also protects them should anyone else have bought it with hostile intent.
Personally I see this as more of an attack on Intel.