First, never underestimate the impact of u$ using their customers as their QA. If you think "Azure" when you say cloud, then--just don't.
Second, while it is correct that the primary benefits of scale accrue to the providers, if there is any meaningful competition at all, a significant amount of that savings will roll down to the customer. If competition is fierce (and it is not), almost all will.
Third, AWS & GCP are set up to provide something that you simply cannot do on your own until you are nearly a household name--reliability. Even four nines is EXPENSIVE. AWS & GCP can give you all the tools for it in a few minutes.
If you don't mind having your website down all day on Black Friday, then you are likely not to need their services at a much lower scale than if you do.
We're moving off Heroku this year. It's going to be interesting to see just how much it is going to cost us to do all of the things that they are doing for us. My prediction is that our costs are going up. It will be worth it, though, because of the added control. That is, we will have a gain in functionality.