This is terrible. Not really just because of Nvidia - which has a lot of problems I've previously commented on the rumors of this [1] - but Nvidia's ownership completely changes ARM's incentives.
ARM created a business model for itself where they had to act as a "BDFL" for the ARM architecture and IP. They made an architecture, CPU designs, and GPU designs for others. They had no stake in the chip making game, and they had others - Samsung, Apple, Nvidia, Qualcomm, Huawei, Mediatek, Rockchip and loads of others make the chip. Their business model was to make the ARM ecosystem accessible for as many companies as possible, so they could sell as many licenses as possible. In that way, ARM's business model enabled a very diverse and thriving ARM market. I think this is the sole reason we see ARM eating the chip world today.
This business model would continue to work perfectly fine as a privately held company, or being owned by a faceless investor company that wants you to make as much money as possible. But it's not fine if you are owned by a company that wants to use you to control their own position in the chip market. There is no way Nvidia (any other chip company, but as laid out previously Nvidia might even be more concerning) will spend 40 billion on this without them deliberately or inadvertently destroying ARM's open CPU and GPU ecosystem. Will Nvidia allow selling ARM licenses to competitors of Nvidia's business? Will Nvidia reserve ARM's best IP as a selling point for its own chips? Will Nvidia allow Mali to continue existing? Any innovations ARM made previously it sold to anyone mostly indiscriminatorily (outside of legal restrictions), but now every time the question must be asked "does Nvidia have a better propietary purpose for this?". For any ARM chip maker the situation will be that Nvidia is both your ruthless competitor, but it also sells you the IP you need to build your chips.
EDIT: ARM's interests up to last week were to create and empower as many competitors for Nvidia as possible. They were good at that and was the root of the success of the ARM ecosystem. That incentive is completely gone now.
Unless Nvidia leaves ARM alone (and why would they spend $40B on that??), this has got to be the beginning of the end of ARM's golden age.
[1]: https://news.ycombinator.com/item?id=24010821