What I find particularly interesting about the ARM acquisition is that it was always hard to see how it was going to continue to grow. The industry is littered with IP companies that get squished by their big customers (see also: Imagination Technologies). It's very difficult to keep a competitive advantage with IP because it'll always leak and people will always catch up, and since all your customers know that the marginal cost of your IP is 0, it's very difficult to extract marginal profit. So there's massive downward pressure on your margins, meanwhile, if your customer is using your IP at a massive scale, they'll either want to pay you a fixed cost for the IP or it becomes cheaper for them to either acquire you or poach all your engineers and replace you (which they naturally want to do anyway since they like owning the IP and having exclusive use). So the natural tendency in this market is for IP companies to pop up with a competitive advantage, grow by selling IP to larger businesses until they finally reach a scale where one of the bigger customers decide to eat their lunch. ARM is well past that point. The only real way to mitigate this is to vertically integrate and become your own biggest customer (like Nvidia) but even then you're going to face challenges (Large cloud providers moving to explore manufacturing their own chips, TPUs etc)
This just seems like many of Softbank's acquisitions where they went in to the deal with a really weird strategy for success and then 5 years later it turns out they aren't smarter than everyone else in the room. It's also not like ARM was some early stage start up at that point, they weren't taking some long bet knowing it had high risk.