Maybe I'd just add one more case here: some users are OK with locking themselves up to AWS by treating it as a platform from the day one and building on top of AWS database/queue/etc services. For those people using EC2 just to run the app code and replacing instances when they misbehave may be a good idea.
This really is the best scenario for AWS (and Azure is also heading this direction somewhat). We spend pennies on the dollar in utilization fees on their services vs what it would cost to implement, maintain, support and scale equivalent services on top of compute.
And to your point somewhere else in here, it is a hell of a thing to try and move away from that platform. Yeah, it's super easy to beat EC2 on cost of compute resources, but really if you're running everything yourself on top of compute at AWS then you're doing it wrong.
Maybe I'd just add one more case here: some users are OK with locking themselves up to AWS by treating it as a platform from the day one and building on top of AWS database/queue/etc services. For those people using EC2 just to run the app code and replacing instances when they misbehave may be a good idea.