4 years, 8 billions. 2 B / year. I don't think that's low. And during this time Arm was also filling its owner coffers. The only real question here is if they filled the coffers at a greater 2B/year or not. My guess is they didn't. Now SoftBank has a lot of cash to acquire more shining toys.
5% over inflation is average. US congress manages 25-30%, but that's on the back of insider trading (technically illegal thanks to a law signed by Obama, but also without much ability to actually investigate -- also law signed by Obama a few months later without fanfare).