It is true that AWS is expensive considering cost of raw hardware resources. However, it is also true that it is very easy to create many kinds of apps and their support is not bad. For example using their "serverless" stuff (lambda, api gateway, static hosting in S3 and cloudfront) it is very easy to make apps with hosting cost almost perfectly matching the number of users so the only upfront capital expenditure is dev time. Another thing that is very easy on AWS is micro service apps (geographically distributed or not) consuming their managed services such as elasticache etc. Finally, automated deployments (allowing for continuous development/continuous integration if one wishes to use it) are also easy to implement quickly...

The price one pays for it is vendor lock in. I wrote many mini-non-mission-critical apps to run on AWS, but I would think twice about putting my main business app in AWS without (costly)backup.

I imagine that for startups it might make sense to create their initial product on AWS and when the product takes off spend some of first revenue making it cloud agnostic.

The benefit of AWS is that a startup can implement a proof of concept quickly without having to worry about managing their own elastic search for example, and once it proves viable effort can be added to escape vendor-lock-in.