What went wrong is that people learned how much NVIDIA pays, figured out they were significantly underpayed, Arm is not a very profitable business so couldn't compete, and the moment it started looking like the NVIDIA deal was not going to fly, top talent started jumping ship.
It may well be that ARM could pay better if they wanted to. But the culture in Cambridge, and the UK, has been to pay engineers pretty poorly. So perhaps they don't yet understand that they need to. Also, salary inversion within the company would be a huge problem.
The UK seriously underpays software engineers, if US companies want to tap into the talent pool its relatively easy to do so especially with remote work. The UK has a serious lack of management talent and a trickle up economy that is damaging everything and the economic situation is not great. Its primed for a big brain drain.