Part of a recent project I worked on involved this very issue, and significant savings are definitely possible. As an idea this is great, there's a gap in the market for this and very few addressing the issue, least of all AWS.

However, there's a couple of little things that may block its wider adoption.

1. It's a big ask for some companies to create any sort of IAM role for an external company or contractor. Even though they send and receive sensitive data from any number of 3rd party APIs, most will be uneasy about IAM access. It's just a hang up more than a concern, but still.

2. Engineering managers either don't understand or don't care about cloud spend. They get their budget at the start of the year, and they grow it based on the previous year. They usually don't have anywhere to put savings later on in the year, and don't want to reduce spend, and hence budget targets, for the following year.

3. I'd half expect your idea to be bought out by Amazon and shuttered. Kudos to you if that's what happens! But it's costing Jeff Bezos another yacht, so he may not like that.