I agree somewhat, but its a little more nuanced. Right up front, you could absolutely get infrastructure setup yourself using things like EC2 etc that would eventually be cheaper when traffic gets high enough but there this does not take into account a few things:
1. The cost of paying the professionals to get this all up and running
2. The delay in getting solutions out to customers to generate revenue since the dev team needs to wait for infrastructure to be spun up first.
3. Ability to handle spikes in traffic. Traditional VM and even container based architectures can take minutes to spin up new infrastructure in response to sudden traffic spikes.
And the comment about "the bills go through the roof" aren't 100% true either. AWS isn't sitting around trying con you into spending all your money. If they were they wouldn't have:
1. Reduced the price of AWS Lambda multiple times since launched
2. Switched from billing for AWS Lambda in 100ms increments to per ms increments, saving everyone using a lot of money.
3. Developing an alternative v2 solution to API Gateway that is faster, easier to use and cheaper per request than API Gateway v1 was.
4. Constantly bringing prices down for all services such as S3, SNS, SQS, etc.
And if you happen to be a high volume user, one chat to your account representative and you get very generous volume discounts across the board, saving you even more.