I think it's a misnomer to call it expensive or compare using a more abstracted service like Fargate vs something more granular like EC2.
If I need a service that lets me prototype, build a product, be first to market, etc. splitting hairs over compute costs seems moot. Not to say it isn't interesting to see the difference in pricing or how AWS quantified how to set pricing of the service.
FWIW, if you watched the Keynote stream, the theme of it was literally "Everything is Everything" and a bunch of catch-phrases basically meaning they've got a tool for every developer in any situation.
One other note: From my experience I'd also argue it's often times easier to migrate from fully-managed to more-self managed services than the other way around. By nature of owning more operations, you make more decisions about how it operates. Those turn into the pain-points of any migration project.