That’s just over 6% compounded annually for a $32 billion hardware company in a 0% interest environment. Those aren’t software numbers but to me it seems like a good investment. And remember - Nvidia was absolutely happy to make the purchase and the industry basically locked that shit down with antitrust litigation threats. The valuation of the company within Nvidia is drastically higher because it gives Nvidia leverage over their competitors and a foothold into other markets (remember Tegra? Yeah, they got their lunch eaten by other ARM SoC vendors).
The real problem with ARM is that they let a lot of the value be captured by their partners who make chips off that design. This is good for the industry, but suddenly 10x more valuable for a single competitor if they manage to acquire it.