I think the only thing you wrote here I would point out is that you seem to vastly underestimate the operational cost of running your own in-house infrastructur

I think the only thing you wrote here I would point out is that you seem to vastly underestimate the operational cost of running your own in-house infrastructure. You're comparing AWS costs to hardware costs, but that's not what AWS gives you, it lets you restructure how you task your entire SA team. You still need them, but they can now work on very different issues, and you don't need to hire new people to work on the things they are now freed up to work on. And again, I cannot over emphasize how much more control over your own time AWS gives you. It's night/day, and discounting that is a mistake.